I can't say that all of the 4th Generation Pokemon are bad, but there quite a few bad eggs in the bunch. My least favorite 4th Generation Pokemon just has to be Burmy and it's evolution line. To start it all off, it's just a very gimmicky Pokemon. Arceus is also like this, with all of those plates, but this is about Burmy! There are too many forms to it, and they are all just very pointless. All of the forms of Burmy are down-right ugly, my least favorite being Trash Cloak. I guess why it's called the Trash Cloak. Burmy is very weak, which is fine since it's not fully evolved yet. But when it evolves, it's still not very good, both of its evolutions. Wormadam still has that horrible form thing, but you can't change it like you could with Burmy. It's still very weak, like Burmy. What's there to say about it? It's just a very bad Pokemon. Mothim is at least a little better. It's still weak like the rest, but it looks more visually appealing.
The Burmy line should have never been created, it's just a waste of space, like some of the other Pokemon. Burmy however, is by far my least favorite 4th Generation Pokemon. I even prefer other un-creative Pokemon over it, like Voltorb.