Q: Why didn’t Newton discover group theory?
A: Because he wasn’t Abel.
Q: What goes “Pieces of seven! Pieces of seven!”
A: A parroty error.
Q: What’s purple and commutes?
A: An abelian grape.
I don’t know why puns should be the main comedy-currency for mathematicians, but they undoubtedly are. There are a few others though:
Q: How many mathematicians does it take to screw in a lightbulb?
Q: Why did the chicken cross the Möbius strip?
A: To get to the same side.
Any offerings welcome in the comments, no matter how groan-worthy…
*Zorn’s Llama. (That’s my addition to the canon.)