I Hate Rude People

My Gf's mom is so freaking rude! Like she's just a mean person. Anytime I try to make a joke it always seems to piss her off and I can't ever just be myself around her. For example the other day my gf was telling her mom she wants a tattoo for her birthday and her mom said no and I was just like "I'll tell you what, for your 18th birthday I'll get you a tattoo of my name 😂" my gf thought it was funny but her mom was just like "stop being smart" and my gf told me later that night that me saying that made her mad. Like I don't know what to say. I can't be myself around her but I'm sure as hell am not gonna change myself to please her. I tried talking to my gf about this and that I don't know what to do because anything I say seems to piss her off and she got mad at me And tells me her mom likes me but that's total bs just from the way she acts. At prom my mom met hers and the next day my mom told me that she thought she was a bitch and didn't appreciate the way she was talking to me. Like wtf am I suppose to do? Tomorrow is my Gf's birthday and I'm supposed to be having dinner with her and her parents but I don't even wanna go...(of coarse I am I just hate being around her parents)