You do know private conversations dont exist in some other realm right? The words you say still matter, even if the consequences change with the audience.
I'd really only give a pass to comedians, because they are writing material for adults who are specifically choosing to go and hear jokes. The same joke said by a comedian is justified easily by context, while a student teacher saying it simply is not.
They didn't have to say it, it wasn't funny, and even if it wasn't a joke and was "venting", there isnt much evidence that venting is a positive thing to do.