It was during the David trilogy. I don't really want to read it just to find Ax's attempt at a joke, but I remember he said something.
The Yeerks had destroyed a column at the building where the world leaders were meeting, and the blade ship was projecting a hologram of it. Ax says something, then Marco says "Did Ax just try to make a joke?". I think that's what happened.
I'm curious about what he said, since Ax trying out humor should be a memorable thing.