Yo mama jokes originated in the southern states when slaves were kept there. Since the slave masters treated the slaves like they werent people, slaves would use yo mama jokes with other slaves. This would make the slaves feel more "important" in the way that they felt higher then other slaves but the other slaves knew it was a joke. Sorry its so confusing but i didnt know how to put it in any other way. I could be wrong though but thats what i heard.