Everyone making fun of Chantelle for her nonexistent booty is MLP-fanon-tier shit.
Leah as a 4chan poster/religious nut/artist is also outta nowhere.
Mewni arranged marriage stuff feels like it was thought up by weeaboos. You know, stuff like "OMG Marco, you held my hand, on my dimension that means we're now engaged, lolz!"
Probably some more but I try not to dwell on that sort of thing.
Actually Marco being called "the safe kid" is the biggest one of all, probably. People wonder why it doesn't come up more, but the only person who ever said it was Principal Skeeves in the first episode, and he's like, what, 60? He's not really an accurate representation of Marco's personal life or the teenage student body. Same episode said Marco was voted "safest kid" by his classmates at one point, but no one else was shown actually calling him "safe kid" in that episode. In retrospect it's like the then-tiny fandom tried to force a meme out of 22 minutes of footage and played it out so much that they managed to convince themselves "safe kid" was once a huge thing.