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Joke judo 🥋 
“And I counter!”
i fucked your mom last night, and I'm not sure if she's alive >

Usually “my mom is dead” means “that joke was no more than 0.001% as funny as you thought it was and now I trust you to only say things that you think are 1000x as funny as they actually are and I don’t want you to ever think of us as friends. I think you’re a bore. Never speak to me again.”
Be funny if you’re going to tell a joke. If you’re going to be a bore, shut up. Just don’t even open your boring mouth. This is the rule. >

Or... Is it not just another burn in a conversation that is typically full of burns?
To me, when done correctly, it just doesn't diss, but rather guilts, the opponent.
Otherwise yea not nice to just say that without clearly saying something like hey could you please not make those jokes around me >

But the idea is that using the "my mom is dead retort" makes the other person feel like an asshole. The point is not "does this invalidate the humor", it is "does this kill the mood, as desired by the person annoyed by the yo mama joke" It is the ultimate comeback because it suceeds in making the joker feel guilty. It's simply good praxis.