> People in thumbnails make weird facial expressions for different reasons

I’ve seen porn videos with a YT-style thumbnail (annoying person making a dumb face in front of whatever is actually interesting), but so far they are very rare.

That would seem to suggest that the obnoxious YT-style thumbnails are mainly driven by children. (Or, at least, with strictly adult-only audiences they don't appear.)

YT uploads with "the face" are by the creator, porn uploads are mainly by random people pirating porn someone else made.

This assumes that market/demographic alone dictates the thumbnail. But context matters too.

I assume this is because the YT annoying dumb face thing is simply less enticing than ... the scenes and "expressions" they can get away with on a porn site. I'd guess a large set of youtube creators would be more risque and scantily clad in their thumbnails if YT would allow it.

So... let's just be glad YT only allows mouths to be agape on the thumbnails you have to scroll past.

No, thumbnails tend to look more like actual thumbnails; a frame in the middle of a video sequence, or figures from a scientific paper, than an excerpt from a tabloid paper.