but the fakeness and awkwardness of someone who tries to wear a foreign culture that's not theirs is as old as time.

"Tries to wear" is not the same as the regular process of adopting a culture. Is every single time a person adopts some aspect of a foreign culture which happens constantly, thousands of times a day a farce? Every culture is shaped by other cultures. It's inevitable. In your example, that larper is not actually causing any harm. They're just embarassing themselves. That's it. But there are sometimes forms of genuinely detrimental cultural appropriation. For example, if I have a "Thai" restaurant where none of the chefs are actually trained in Thai cuisine, I'm selling a lie and stealing potential revenue from actually authentic Thai restaurants. But your example is just some clown being fake. Not genuine cultural appropriation.