Laughing at a cruel joke is a vote. Staying silent is an abstention that still counts.

A joke at someone’s expense needs an audience to become a group event, and everyone present is part of it. Laughing supplies the fuel. Saying nothing does not remove you either, because the person being laughed at is counting faces and yours is in the count. That does not make you the one who told it, it just means neutrality is not really available.

You do not have to make a speech. Not laughing is enough, and a flat remark that it was harsh does more than people expect. If that is too much in the moment, find the person afterwards and say one plain thing to them.