One good joke to make fun of someone's mother is, "your mama is so ugly that when she ran away from home, her parents called the dog catcher." Another joke to make fun of someone's mother is, "your mama's breath is so bad, it's like her tongue farted."
Comedy Central is a good source for finding jokes about someone's mother. It has jokes that focus on intelligence, for example, stating that mama saw an airport sign that said "airport left," which caused her to turn around and leave, or stating that mama tried to drown her own fish.
Other jokes about someone's mother can focus on being poor, stating that she is so poor she gets breakfast from the outside bird feeders, or that she hangs toilet paper out to dry.
Some jokes focus on the age of someone's mother, stating that she used to work with Fred Flintstone or that she has a Bible autographed by Jesus. Another one that mocks a mother's age states that it takes an hour for her to make Minute Maid.
A joke that teases someone's mother for being hairy states that she is so hairy her knees have bangs. To tease her height, a person could say, "your mama is so short, she has to pole vault into her bed at night."