It is funny to watch the sign language interpreter try to keep up, especially if the speaker is using lots of words outside of the ASL vocabulary. I saw the Capitol Steps once and they were using a local sign language interpreter who wasn't familiar with their performance, especially the "Lirty Dies" routine where a lot of the humor is switching letters is words so the signer has to spell them out. Part way through the performer stopped, looked at the signer and said something like "I think her fingers just fell off". The crowd laughed and she then had to sign that joke too.
Jeff Dunham and Peanut have a bit on having had an ASL interpreter at a performance. One of the funniest bits I have ever seen - gasping on the floor funny.