"Yo' mama is so fat, her belly button gets home 15 minutes before she does" and "Yo' mama so ugly, people go as her for Halloween" are some yo' mama jokes. Another is "Yo' mama is so fat, I took a picture of her last Christmas and it's still printing."
The actual history of the yo' mama joke may not be fully known, but they rose to popularity in the 1990s through hip hop and rap culture and on sketch comedy television shows with multi-racial casts. They are typically considered in the African American comedy genre, but many cultures have adopted them. Yo' mama jokes can be funny, but they can also can be rude and offensive when using inappropriate or mean content.
While the actually yo' mama joke term may be a fairly recent development, as of 2015, the act of making fun of someone's mother for a laugh is almost as old as language itself. The first documented joke about a person's mother was found on tablets carved in ancient Babylon over 3,500 years ago. Archaeologist J.J. van Dijk found the original tablet in 1976. The stone contained six riddles, and while some things are probably lost in translation, one of the riddles included the words "beer," "sexual intercourse" and "your mother."