If you don't notice it, you may be part of the problem.

Don't get me wrong, Undertale was a great game, but it wasn't a perfect game. The characters are for the most part pretty bad, the story wasn't all that great by itself, and the gameplay mechanics were refreshing sure, but simple. What Undertale had was charm, cheap laughs, and a refreshing system of morality in combination with refreshing gameplay mechanics.

There are a few different types of people who play Undertale (or any game in general). Normal people who just enjoy the game (or not) and talk about it once in a while when it's relevant, people who love Undertale, who maybe play into the fandom a bit and make some Undertale related content once in a while, but overall don't let it consume them. And then there's the cancer, who go around spouting Undertale everywhere, making content that is hardly inspired, and just overall continuously ******** or talk about it. And there are A LOT of cancerous people like that in the Undertale fanbase.

Undertale fans tend to interject their "fandom" into every little ******* thing in the known universe. Obscure references that only make sense in an incredibly tangential way. People roleplaying as the characters all over the Internet (which is about the cringe worthy thing you can do). Interjecting and ranting about legitimate criticisms of the game at EVERY opportunity they get. People focusing on really, REALLY meaningless aspects of the game while ignoring what makes the game so great, really not doing the game any justice.

Not saying that Undertale fans are getting as bad as say FNAF or Homestuck, but they are pretty bad in the grand scheme of things.