Effect is a cycle that any form of entertainment media community can fall into.
Essentially, the community will COMPLETELY over rate the show/game/book/movie/etc., and defend it as if it is their life. The community will almost always turn the lovable, funny characters into cringe, and rapidly spread throughout the internet.
The strange thing is that the only outlets to suffer from The Undertale Effect are those from something that by itself is great. The show/video game/movie/etc. is usually great, and has many redeeming qualities but it is overshadowed by the atrocious and toxic fanbase.
Quite obviously, this is based off of the worst case of this ever happening yet - Undertale. I won't get into it now, but the smallest peek into the (dead now) community will tell you.
This has infamously happened to: Steven Universe, Five Nights at Freddy's, Minecraft, and unfortunately even Rick and Morty.
Right now, Rick and Morty has fallen into the Undertale Effect. Just look at Musical.ly, the fan art, the cancerous reddit copypasta, etc.