I'd genuinely love to have Undertale songs in Smash, but nothing more, really. A stage is probably as far as I'd go without being baffled, and I'd still find it a little confusing (although I do know the game is popular in Japan.)
It really isn't.
There's far better reasons to dislike many fandoms, even in the case of bronies. 'Cringe' is not and never will be a valid reason, it's just childish as it literally doesn't amount to anything other than "haha, you're weird!"
We're not school bullies, and we shouldn't act like them.
Especially in the cases of Undertale and FNAF where you're literally just laughing at children for enjoying something. Isn't this the same thing Nintendo fans have had a problem with, people judging you for liking cartoony games by a "kids company"?