The community hasn't had as much time with the Pokémon from newer generations as they've had with the old ones. It takes time for a community consensus on favorites to emerge, or for a Pokémon's favorable reputation to develop. And newer Pokémon are not just behind the older Pokémon on a timeline of establishing themselves—they're having to establish themselves and stand out alongside older Pokémon that have already established themselves in a lot of fans' hearts and minds.
So it seems to me that newer Pokémon will, in general, always seem lesser when compared to the Pokémon that came before—and fans will wonder, "what's wrong with the new designs? They must all suck." And they could suck, that's definitely a possibility. There are a few designs that I think are terrible. But I think that Pokémon fans are too quick to declare new designs as "bad" when, sometimes, you just need to give it a bit.
Even in my own time playing, I've found that my opinions on newly-introduced Pokémon has almost always gotten better over time. There are tons of designs from Gen 5, for example, that I thought at the time looked like dogshit, unserious, retarded, not fitting with the established style of the series, an insult, ruining everything, franchise is doomed, etc. And now I actually like a lot of them.