I always figured it was an engagement optimization thing—there were people mass producing content using popular characters and just throwing tons of stuff at the wall, and the ones that veered unsettling/bizarre wound up getting lots of engagement so they kept doubling down on it. That kind of feedback loop is certainly responsible for many other curious traits of online content that is circulated in algorithmically-curated feeds.

The tighter the loop between content creation (e.g. when you can generate unlimited content essentially for free) and the ability to measure its success (engagement), the more social media becomes a sort of genetic algorithm for optimizing content to be the most addictive possible at the expense of any other attribute.