Can I call my girlfriend fat as a joke?
That does not sound good. How will your girlfriend feel if you call her fat?
I don't care, it's just a joke
I know you think it's a harmless joke. But mean jokes aren't really funny especially since you say you don't care how it makes her feel.
How about you just get over it and leave me alone. This is between me and my girlfriend.
I am just trying to help you and your girlfriend out. If you make a joke about their expense, your relationship will suffer and it might not recover.