Mario doesn't really have a garbage fanbase, mostly because very few people get obsessed over Mario the same way they would over something like Sonic.
The Sonic fanbase is terrible for many reasons, the biggest reason being that it's split into multiple extreme camps: the deviants (erotic roleplayers, porn artists), the depraved (vore and scat enthusiasts), the mentally disturbed (Sonic Passion), the retrofags, the Adventurefags, the AllSonicgamesaregoodfags, the canonfags, SatAMfags, comicfags etc. Even then, there's plenty of overlap between camps so there's always a lot of diversity in the shittiness you encounter. Mario's fanbase is a lot more unified in compassion, because there's not really a lot to talk about besides the games. It's also not nearly as depraved.