It's the "Voldemort" of comedy - you'll rarely hear a comedian actually call another comedian "not funny." Between fear of it getting back to "that hack" and ruining your career when he makes it before you do and fear of the instant karma that someone's saying the same thing about you right now, it doesn't happen in mixed company. Instead, we've developed a code that let's the person you're talking to know you mean "I don't think you are/he is/she is funny" without having to say the actual words.
Here's a sample, got any of your own I missed?
"You look really comfortable on stage"
"She's the nicest person"
"He does well with audiences"
"She produces really great shows"
"You know, he gave me a spot on his show...."
"I like that idea"/"You've got some good premises there"
Cowardice or just being polite human beings? Okay, probably not the second thing, we are talking about comedians.