Gen V had really ugly, blurry sprites with horrible tweening animation that made them look even worse, especially if you were playing on a 3DS XL. With that and the generally dark, unappealing color schemes on most of them, it's pretty easy to see why people have a harsher view on a lot of the Gen V's designs than they really deserve. I quite like a lot of the Gen V Pokemon, though I didn't warm up to some of them until I saw them in XY.
That said, Gen V DID have some truly hideous designs. The Genies are the ugliest fucking things I've ever seen, even in Therian Forme, and the Ponies don't far much better, either.
