Some Pokemon like Eevee have evolutions that act like an actual evolution, some creature adapting to certain living conditions. However, most Pokemon don’t actually evolve, they just grow up; hence baby Pokemon. Bulbasaur isn’t adapting to a new environment or anything it’s just getting older, thus the bud on its back blooms and its body grows. Does this bother you at all, or do you not mind it?
Well, Pokémon evolution is sufficiently different to real-world evolution anyway that details like whether it’s ‘adaptation’ or not kind of go over my head. Darwinian evolution has no effect on individuals. Organisms cannot ‘evolve’ within their own lifetimes. Only populations can evolve. What Pokémon are doing – dramatic change within the lifetime of a single creature – is really metamorphosis; it makes more sense to compare Bulbasaur to, say, what a cicada or dragonfly does. Evolution is a bit of a silly thing to call it, I suppose, but I think I’ve been desensitised to it over the sixteen years I’ve been playing Pokémon.