React in kind.

“That person looks ugly/dumb/etc.”

Make confused/disappointed face, don’t laugh. “Uh… Okay, kind of mean.” Then move on.

Most people behave to impress/entertain their friend group. Not sure if they’re your friend or not, but that’s been successful for me. It’s not to attack them, but just to show them that that behavior isn’t going to help your opinion of them.

If they don’t care, then that’s a choice for you to make about what you’ll tolerate.