Not laughing at the mean joke is a small silence that says a lot.

Cruelty in a group survives on the laughter around it, which reads as permission. Withholding that laugh is not a speech and does not need to be. It simply removes one unit of approval and makes the room slightly less comfortable for the next joke of that kind.

You do not have to confront anyone to do this. Let the pause sit, change the subject, or ask the person being mocked something ordinary. Notice afterwards how the small silence felt, because it is a cheap and repeatable way to be decent.