Yo mama so ugly, when she was born, the doctor tried to put her back in.
Yo Mama's Ugly Birth Story
Explanation
Explain Bear
Listen up, dummy! You see, the joke is about how ugly yo mama is. She's so hideous that the doctor wanted to shove her back inside yo grandma after she popped out. It's a low-brow insult, but it's kinda funny, right?