Part 1: Names the translators didn't even bother to change.
Pachirisu. It's this generation's Pikachu, i.e. a cute Electric-type rodent. We've already had Pikachu & Raichu in Red/Green, Pichu in Gold/Silver and Plusle & Minun in Ruby/Sapphire. Pachirisu is a squirrel. It also doesn't evolve (yet), so it's almost completely useless.
Hippopotas. It's a hippopotamus. Like you couldn't have guessed that.
Skorupi --> Drapion. In katakana, these names are written as Sukorupi and Dorapion, but this is because all Japanese syllables need to end with a vowel sound, except for m/n. Guessing that Skorupi is a scorpion is a no-brainer. It's less clear that Drapion is also a scorpion, until you realise that it's common for the evolved form of a Pokemon to take its name from the other half of the name of the real-life animal it's based on. Hence SKOR-upi, Dra-PION.
Riolu --> Lucario. Lucario is the hero of the 9th Pokemon movie, and the big surprise was that it isn't a legendary Pokemon (a Pokemon there's only one of in the game). It's a doglike thing that walks on two legs. I have no idea where the name came from, but it's easy enough to rearrange the syllables of rio-lu and add "ca" to get lu-ca-rio, showing the connection between them.
Weavile is the new evolution of Sneasel, a Pokemon that desperately needed an evolution because it has a cool type (Dark/Ice) but was too weak to use before. It's a weasel, and presumably a vile one (again, snea-SEL, WEA-vile).
Dialga, Palkia, Manaphy. Dialga and Palkia are the legendary Pokemon in Diamond and Pearl respectively. Their names are uninspired corruptions of the gemstone names. Manaphy is a legendary that you can't get until after you've finished the main game and upgraded your Pokedex and done a special mission in Pokemon Ranger (or cheated to obtain the code some other way, all iirc as I don't have a functioning internet connection right now). It is not a manatee, but several people have sung the Dugong song substituting Dewgong and Manaphy.
Part 2: Names they changed even though the Japanese name was perfectly fine.
Bippa is a beaver who's a bit stupid, and not terribly powerful. In English, they decided to play up the "stupid" angle and came up with Bidoof, as in beaver+doofus. I wouldn't mind this if it sounded like a name, but it doesn't to me, it sounds cut-off halfway. "Bidoofa" would be better imo.
Korink is an Electric monkey-type thing. Unlike Pachirisu, it evolves twice, becoming progressively more powerful. Korink would have been a perfectly acceptable English name. But no, they went for... Shinx. Even though it's a monkey, not a sphinx. Even though Shinx sounds more like "shit" than "shine" (different sound of the middle "i"). Its evolved forms are called Luxio and Luxray, both of which sound like domestic appliances to this child of the 70s. "Get the new Ford Luxio - the affordable luxury car for your family." "Buy the brand-new Luxray vacuum cleaner now, and get a free dust bag!".
Rozureido is the evolved form of Roselia, a plant Pokemon that holds two flowers in its hands. In English, it has become Roserade. Is that Rose-red or Rose-raid? Or somehow trying to be both at the same time? The new pre-evolution also suffers from a naming problem: in Japanese, it's Subomii, which is cute even if you don't understand what it means. In English, it's become Budew, which is... ew.
Mitsuhoney is a strange little section of honeycomb that can, if female, evolve into Beequeen. Perfectly acceptable English names, you think. Well, Mitsuhoney has become Combee, which sort-of works (it's three hexagons combined to form a bee). But Beequeen has attempted to be clever, and used the Latin word for "wasp" to become Vespiquen. Yes, Vespiquen. Not Vespiqueen, because that would make sense, and heaven forbid we do that in a Pokemon name! I don't even get why it's Vespiquen, because the old 9 letter restriction on names that made Farfetch'd named that way is obviously no longer in force, or we wouldn't have Hippopotas, Kricketune or Ramparados in this very generation!
Buoysel's name conjures up a weasel floating in a rubber ring, or buoy. In English, it has mysteriously become Buizel. Why? It's been rumoured that American children are too stupid to know how to pronounce "buoy", but this seems rather unlikely considering the US has spelling bee contests as a regular part of schooling. Possibly they changed the name to make it more fake-Japanese and hence cooler to the kids of the naughties... I dunno.
Doomiraa is another name that gives a clear impression of the Pokemon: Doom+mirror. In English, this has become Bronzor. I think they think it's a shield (made of bronze) rather than a mirror, but yeah: Doomirror would be a good name, Bronzor sounds like Skeletor and the rest of the He-Man universe. Its evolution, Bronzong, is presumably based on bronze+dong, and is as hilarious as any bronze dong can be. (Especially when you are reminded of the giant ride-on bronze dong in the 24 hour "adult entertainment" store that some people visited during the last alt.polycon - ahem!).
Perap is a parrot that can speak human language. Its gimmick is that you can say words into the DS microphone, that then get played every time you launch your Chattering attack. I don't know whether some sort of voice recognition software exists to stop the Perap shouting swear words at its opponent, but it seems unlikely. I also think having the DS talk at you in your own voice would get boring after, ooh, the first 10 times. Anyway, the Japanese name is obvious: parrot+rap. We all thought it was going to become Parap in English. But no - they went for Chatot. This may be to avoid confusion with Parappa the Rapper (another game), but I doubt it.
Part 3: Some slightly silly names.
Munchlax was the first 4th gen Pokemon to be revealed. It's the pre-evolution of Snorlax, a Pokemon that does little more than eat and sleep, and is often used as a plot device to block paths temporarily. For some reason that I still don't understand, "Snorlax" does not sound like a pharmaceutical product of any sort, whereas "Munchlax" sounds like a laxative.
Drifloon --> Drifblim. It's a balloon, that evolves into a bigger balloon. So far, so good. But what's wrong with spelling the names properly and calling them Driftloon and Driftblimp? Even "Drifblimp" would be better than "Drifblim", a name that is half-finished twice (making it only quarter-finished in total?). Gah.
Buneary --> Lopunny. It's a bunny with ears, that evolves into omg, a human-shaped bunny-girl. Some commentators are already calling it the "fan-service Pokemon" (as in, bad "erotic" furry fiction will be written about this creature). Buneary is cute, its evolution scares me. But it's the evolution that has the humungous ears and should be named accordingly, while Lopunny just makes me think of puns, not rabbits or bunnies or lapins.
Piplup. The Water starter is a baby penguin, called Pochama in Japanese. It's sooo cuuuute. So why does it have a name that sounds like water splashing into a drain, or poo splashing into the toilet? Its evolutions are Prinplup and Empoleon, as in prince and emperor - these are better, though still atrocious.
The Fire starter, called Hikozaru in Japanese, gets the English name Chimchar. Apparently, this sounds like a Fire-type monkey. Apparently, they didn't have any Brits on the team who could have told them about the PG Tips adverts featuring chimps, and the fact a "cup of char" is Northern for a cup of tea. Hence, whenever I see "Chimchar" in print, I think it's going to be a monkey holding a cup of tea and am all confused by it instead having its arse on fire.
Part 4: Names I actually like!
The evolved form of Aipom (a monkey with a hand at the end of its tail) has a split tail with two hands. It's called Ambipom, like ambidextrous. This is cool.
Chimecho is a bell Pokemon from Ruby/Sapphire. Its pre-evolution is in Diamond/Pearl, and is called Chingling. I think this works - the new English name keeps with the onomatopoeic theme of its evolution, and sounds like a little bell.
There already existed a Pokemon called Chansey which evolves into Blissey. (They're pink blobby things, so the name is mostly irrelevant - although, in Ruby/Sapphire, it's a happiness evolution i.e. the Pokemon doesn't evolve at any fixed experience level, but instead when it likes you enough.) The new pre-evolution is called Happiny. It's a slightly odd name, but it's in accordance with the naming theme, and sounds like a name.
Gible --> Gabite --> Garchomp. Without knowing anything about these new Pokemon, you could guess that their mouths are important. You'd also get the evolution order: a nibble is smaller than a bite, which is smaller than a chomp. The "Ga" on the beginning could mean almost anything, though they happen to be reptilian. I also like their sprites - they're clearly based on the same idea but getting progressively more complicated. I'm sure Pokemon purists would argue there's not enough difference between Gible and Garchomp, but I don't like change for the hell of it. (In contrast, see from Gold/Silver: Remoraid, a blue remora fish, that involves into Octillery, a red octopus with a bazooka cannon in its mouth. Uh... yeah.)
Shieldon --> Bastiodon. The names tell you that these Pokemon are heavily-armoured (shield, bastion), and you could guess that they're probably Rock types. Also the names seem related - a shield is a smaller amount of armour than a bastion.
Shellos --> Gastrodon. These Pokemon are Water-type snails that can be found in both grass and water locations. They have different sprites depending on whereabouts you caught them, but I believe this is purely cosmetic and just a way to get you to bother catching Yet Another Water type. I'm not so keen on Shellos (it has a shell - wow), but I rather like Gastrodon with its implication of gastropod.
Bonsly is one of my favourite Pokemon of this generation. One of the first ones to be revealed in the anime, it's the pre-evolution of Sudowoodo. Which is a Rock-type, even though it looks like a tree. (Supposedly, it mimics a tree to avoid being caught.) Bonsly is a baby tree in a pot, and BONS-ai + cute ending is a decent enough name.
Torterra is the final form of the Grass starter, a turtle. Its pre-evolutions are Turtwig (ugh) and Grotle (grotty). But Torterra conjures up an image of a huge tortoise that carries earth on its back. It contains both "tortoise" and "terra", and at a stretch, "terrible". This fits the Pokemon remarkably well.
Spiritomb. It's a reasonably unimaginative name for a Ghost/Dark type Pokemon, simply spirit+tomb, but it hasn't been used before. The only question is whether it fits the new Pokemon that's been given the name - a purple/green blobby cloud OF DOOM.
Part 5: The jury is still out on...
Starly --> Staravia --> Staraptor. It's a starling that evolves into a bigger avian and then into a raptor. I think it works, but I'd need a bit longer to be sure.
Kricketot --> Kricketune. It's a fairly useless cricket that evolves into a slightly less useless bigger cricket. Some of these crappy-in-battle Pokemon are put in because they're good in contests, a part of the game which is hugely popular in Japan but much less-so outside. Personally, I always enjoyed the contests (in particular, taking the ugliest Pokemon I could think of and raising its Beauty stat so high it would sweep the Beauty contest), but with my game being in Japanese, I haven't played them at all this gen. I'm told they've changed dramatically and are much more fun now. With all this in mind, it's possible that Kricketune has some sort of affinity for music and is good at dancing.
Stunky --> Skuntank. It's a skunk that evolves into a bigger skunk. I'm guessing the name is supposed to be stinky+skunk. But Stunky sounds odd, especially if its evo is Skuntank. But then, it couldn't be Skunty, could it? Skunky would have worked, but doesn't have the t in... but why does it need a t anyway?
Burmy is a Bug-type Pokemon - a bagworm that can evolve into a moth if male, otherwise it becomes a bigger bagworm. It seems like a totally nondescript name - Bur + cute ending. Wow, that took a lot of effort! Probably they realised they couldn't just go with "Wormy", so changed the W to a B for bagworm or Bug-type. Unless, of course, Burmy turns out to be an amazing Bug/Fire dual type (impressive, as Fire is Super Effective against Bug types - it would be like a Grass/Ice combination), in which case it's like "burn", and not quite so silly.
Cranidos --> Ramparados. There is nothing particularly wrong with either of these names (Cranidos has an armoured cranium, Ramparados must rampage a lot). But in previous generations, Pokemon names ending in -dos have been the second of that type, e.g. Gyarados, Zapdos. It seems strange to break that convention now.
Purugly. It's a hideous, fat, Persian cat. (I've separated "hideous" and "fat" as it's possible for both animals and people to be hideous but not fat, or fat but not hideous.) Its pre-evolution, called Glameow in English, is a relatively normal ("glamourous") Persian cat, but Purugly is hideous, and deserves the "ugly" part of its name. I'm just not sure about the whole concept of having an "attractive" creature evolve into an "ugly" one, and then labelling the evolution as "ugly" to boot. Is it saying that if you gain muscle and strength, you automatically turn ugly? And I don't really care for having Persian mis-spelled as "Pur", either.
Part 6: Names I have nothing to say about.
I haven't bothered to comment on 23 names. This is either because we've had so long to get used to the name that I can't remember my original thoughts on it (Mime Jr, Mantyke) or because I genuinely have no opinion (Monferno, Infernape, Bibarel, Wormadam, Mothim, Floatzel, Cherubi, Cherrim, Mismagius, Honchkrow, Hippowdon, Finneon, Lumineon). In some cases, I think the name they've gone with is a little weak, but I can't think of a better replacement (Croagunk, Toxicroak, Carnivine, Snover, Abomasnow, Uxie, Mespirit, Azelf). I will freely admit that I expect Toxicroak, Carnivine and Abomasnow to grow on me :)
Finally, consider yourself warned that in 3 hours of boredom while my net connection was down, I managed to write 2609 words of utter drivel about Pokemon. Imagine what my brain could do to the world if only I had the motivation! ;)