> As an aside, I've never understood that joke about Clinton. Was it a reference to something in particular?

It's a reference to his defense of saying he wasn't lying with the phrase "there's nothing going on between us" when questioned about his relationship with Monica Lewinsky.

He explained that this was true because they were no longer seeing each other at the time of questioning, but the way he said it was striking. "It depends on what the meaning of the word 'is' is. If the—if he—if 'is' means is and never has been, that is not—that is one thing. If it means there is none, that was a completely true statement"

I think it's easy to see why people would latch on to that and make jokes about it.