I'm sure in a particular context, with a friend that cares and understands, that poking fun is okay to do (so long as the person that stutters consents to this). I don't think this is what OP is referring to, though. Some people are jerks and think this is okay to do to anyone, and not in a way that is genuinely in jest. Some people are just downright cruel, as I'm sure we've all seen.