I'm planning on making a joke about someone's girlfriend.
Why are you going to make a joke about her?
I'm jealous. I wish she were my girlfriend.
It's understandable to be jealous of someone, but you shouldn't act in anger and spite because you can't get what you want. You'll come off looking petty and people may not want to associate with you.
I'm going to say it anyway.
If you go ahead with your plan, then you're going to upset that person and their girlfriend. It would be better for you to move on and find someone new to focus your attention on, instead of cracking jokes about someone's partner.