The blowback against Geno is just simply just the fact that he has a loud and kinda annoying fanbase inspite of just being a one-off party member from a two decade year old spinoff game.
Like you look at K. Rool, Ridley and Banjo-Kazooie and they still had merit outside of just being requested. The former two are (or at least, were, in K. Rool's case) reoccuring villains in their respective long running franchise and BK were the main protagonists of their own line of games that used be a successful Nintendo co-owned franchise. Obviously they needed fan demand to give them that final push to be in Smash, but their inclusions still felt natural.
Objectively speaking, Geno isn't any more significant (he's probably even less important, even) than characters like Waluigi, Midna, Ashley, Krystal, Skull Kid, etc and the most they can get are assist trophies. So naturally, yeah, people are gonna get annoyed at other people constantly yammering about wanting this really minor character in Smash.