Laughing along at a cruel joke is still a vote. Your silence got counted too.

A cruel joke needs a room to work, and the laugh is what tells the person telling it to keep going. Whoever it lands on is not counting who meant it and who was just going along. They are counting faces. That is why going along feels so harmless from the inside and lands so heavily from where they are standing.

You do not have to make a scene about it. Not laughing is already a full sentence. Changing the subject, checking on the person after, or saying you are out of this one all work, and none of them require courage you do not have.