I'm curious what makes one game's fandom bad, but not another
I've seen loads of badly made/cringey videos of source games made in SFM/Garrys mod, including "research" videos, people constantly talking about league of legends (and Lol research). Crossing their content over with other content, etc. Just generally doing the same things that people complain about FNaF or Undertale fans doing. But I've never heard complaints about these games.
But as soon as someone sees a single blocky character, animatronic, or naked skeleton, they run around complaining how Minecraft, FNaF, and Undertale fans are everywhere and there's so much research . When really they're just doing the same thing as everyone else.
The problem with MLP specifically is that a large majority of the audience are in fact children. And there was a problem (not sure if it still persists) where searching for MLP related things would result in some amount of unflagged research, because a number of the older fans couldn't seem to find the decency to upload things properly. The fact that it exploded very fast so that there was much, much more content to go through made the problem worse, most likely. I'm not personally a witness to this but I have seen some people (MLP fans specifically) bringing up this concern a few times in the past.
Though research in a fandom doesn't exactly make something cancerous. There's research of everything. It just seemed like the MLP fandom was the most irresponsible when it came down to keeping mature content where mature content goes.
As for general 'cancerous' things go, these things really do come and go. It's not just 'hating the popular thing' at the time. When Minecraft was first emerging, or when specific youtube uploaders came around, there were a lot of annoying behaviour that resulted from it. Including but not limited to "this is a minecraft ripoff" "creppers!!!" "budder sword"
MLP wasn't nearly as bad, but the 'ponification' of every existing thing was pretty excessive.
FNAF isn't a widespread problem. It's 'cancerous' activity sticks to a few select places. Like LittleBigPlanet, for a sad example.
At the very least, Undertale is original. While people do annoyingly 'act in character' a little too much and continuously recite a few specific lines from the game, there's at least content/context behind it. And that's only a small part of what we see in the fandom.
League seems contained. I haven't ever saw anyone going into random threads and Dravening them up, or anything of the sort. League typically stays in League.
Garry's Mod/SFM are unfair claims. They're mostly used for producing content, so they're more of tools than they are fandoms. They're typically used to create content for other fandoms. So if you're seeing TF2 research where you shouldn't, that's a TF2 related issue. You can't compare, for example, TMNT to Adobe Flash.