The series got away one time before with a Pokémon that resembled a Pokéball but I don't think anyone can agree they can get away with it twice. The joke of running into a Voltorb that was mistaken for an item was a great gimmick in Generation I, but going for the same joke again some years later just isn't going to fly. I can't exactly say why Pokémon thought Foongus and Amoongus were necessary other than attempting to make Gen V rival Gen I. The idea that mushrooms just happen to grow to look exactly like Pokéballs feels a little bit like a stretch even in the Pokémon universe. This is indeed a case where attempting to copy a success twice seems like milking a dry cow.