Memes

Yep. It's been a thing for a long time now, at least since 2001, but "it's just a joke" became a way for bigots to disguise their hatred, and was picked up by "edgy" teenagers in places like 4Chan...which would later reveal as they got older that they were actually just bigots themselves all along or the joking had normalized that mentality in them to the point where they started believing it. It's definitely not just an American thing, but Republicans in the US tend to be extremely sensitive and volatile emotionally (despite calling everybody else "liberal snowflakes" for how easily offended they claim they are) while being very emboldened by the events of the past decade or so to openly spew their hatred and bigotry.

There's a great stand-up bit about it by James Acaster from 2019 called What's Wrong? Too Challenging For You!?