A mad case of propaganda, I'm guessing. As in, people hating on the fans more than the character himself, which is a stupid reason to hate a character to begin with, but that hasn't stopped anybody before. These people probably only saw one or two Geno fans acting out of line, and then proceeded to extrapolate that type of behavior to the entire fanbase. It's incredibly stupid, but it's what happened to the Ashley fans when one (1) person lashed out on Twitter about Castlevania content getting in the game when Ashley was still an AT. It sucks, but the misbehavior of one person on the behalf of a fanbase can reflect badly on the entire fanbase, which is why we all need to do our part in acting 100% mature and civil going forward. Otherwise, we just give the haters more ammo to use against us.
That, and popular requests always get the most hate for some reason. K. Rool and Ridley were common "never-ever" scapegoats before they got confirmed, so when BTFO-culture hotspots like 4chan and GameFAQs needed a new one, they went for the next most popular and targetable candidate, that being Geno.
It's a tough call for sure, but probably the entire stretch from Booster Tower to Marrymore. Booster is such an entertaining character, and I have such fond memories of climbing up Booster Tower as a child and hiding behind the curtains to evade the equally entertaining Snifits.
And yeah, "Yo Smithy!" is pretty great, too.