Yo mamma’s so fat, she sat on a rainbow and made Skittles pop out.
You win the thread! 
Best one so far.
You momma’s so fat, when she goes to the beach, Greenpeace keep dragging her back out to the ocean.
… when she goes to the zoo the elephants throw her peanuts.
… her high school graduation picture was an aerial photograph.
… I ran around her twice and got lost.
… when she goes to a restaurant, she doesn’t get a menu, she gets an estimate.
Yo mama’s so fat, her twelvsie looks like a onesie.
Yo momma so fat, her picture on facebook is still downloading.
Yo mama’s so fat, she makes Debbie Wolowitz look like Twiggy.
Yo momma so fat they had to change the labels to “one size fits most”.
Yo momma so fat little kids at the beach pay her to sit in her shade.
Yo momma so fat it takes five cents to weigh herself on a penny scale.
Yo momma so fat you gotta hug her in sections.
This one cracked me up!!! 
Yo momma so fat that when you were born it took them 3 days to find you.
That’s not really a yo momma joke. It’s making fun of two real people.
Yo momma so fat, Spanx signed a non-aggression pact.
Your momma’s so fat, when the Judgement Day comes and we’re all carried up to heaven, Jesus is gonna look at her and start crying.
Yo mama is so fat that her cereal bowl came with a lifeguard.
Well, one real person and one fictional person.
… when she wears a yellow raincoat, little kids try to ride her to school.
… her belt is an event horizon
…they’re one cleared neighbourhood away from being a planet.
Ya mama is so big and fat that she can get busy/With twenty-two burritos, but times are rough/I seen her in the back of Taco Bell with handcuffs
Yo mama so fat she makes Big Bird look like a hummingbird.