I'm going to tell my girlfriend she's not funny
Why would you be so mean to your gf?
I want to teach her how to be funny.
It's mean to tell someone they aren't funny. You should be nicer to your gf.
Why? What's there to lose?
She would be upset that you brought that up and would be insecure about it. Even if you do teach her your humor, it would be forced and uncomfortable.