Yo mama is so ugly, when she looked at the mirror, I cracked up.
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Explanation
Explain Bear
Listen up, buttercup! The joke is that yo mama is so ugly that even a mirror can't handle her face. When someone looks in a mirror, they expect to see a normal reflection, but yo mama's ugliness is so extreme that it causes the mirror to "crack up," meaning the person cracks up laughing at the ugliness.
Andrew
You’re so ugly when you looked in the mirror that you crack it