I'm going to joke about my girlfriend's parent's involvement in MLMs.
Is this because they approached you to join their scheme?
They sometimes pitch me and my friends about pyramid schemes and we laugh at them.
I understand you get annoyed with them involving you guys in the pyramid scheme. But they're your girlfriend's parents so you should not make fun of them. If MLM is their income source, you should not make jokes about it.
I don't care. I'm going to tell everyone around, especially her neighbors. They are all losers anyways.
MLMs can definitely be sketchy and your girlfriend's parents pitching you is understandably annoying, but you shouldn't make fun of them. This is how they make money and don't you think their feelings would be hurt? Rather, set boundaries and kindly ask that they stop pitching you.