Yo' momma so fat her belly button has an echo.Instead, I would say:
Yo' momma so fat that she is at serious risk for Type II Diabetes, cardiovascular disease, and various respiratory problems. She probably won't live more than 3 more years.This is not exclusive to jokes demeaning mothers; they can be used for all afflictions of life.
You so ugly that your mom fed you with a slingshot.Unfunny/Super-funny alternative:
You so ugly that you will immediately be judged poorly when meeting people. This will severely limit your job/friendship/relationship opportunities. You'll probably die poor and alone. I'm sorry.Traditional insult:
Yo' momma so stupid it took her two hours to watch 60 Minutes.Unfunny/Super-funny alternative:
Your mother has severe mental handicaps. You should probably put her into some sort of home and prepare yourself to live life as an orphan. Hopefully, she won't be abused and forced to live in her own filth. Good luck with that.With a little effort, the yo' momma joke can be used to change the way we look at the bodily/facially/mentally disadvantaged. They need help. Remember, everyone is someone's mother/daughter/father/brother/son.
And they, too, have mothers to use as targets for our own amusement.