I know it kinda sounds retarded, but i’m hungry for entertainment.
Ill start.
Your mom is so stupid, she tripped on a wireless connection.
Here’s mine (:V):
Your mom is so ugly, when she fought Scorpion, he said “STAY OVER HERE!”.
Your mom is so fat, she’s a Skyrim map.
Your mom is so ugly, she’s the reason why Sonic goes fast.
Yo momma so fat, she needs a passport to go from one room to another.
Yo momma so fat, when she falls off the bed she falls off both sides.
Yo momma so fat she needs a map to find the toilet.
Yo momma so fat, when she sits on an iPhone she turns it into a flat screen TV.
These are the only ones I could of the top of my head. 
Yo mama so nice that she donated a kidney to an orphan and saved his life.
Real American hero.
Yo mama so fat, she’s at an increased risk for developing cardiovascular problems.
Yo mama so stupid, her IQ is 16 standard deviations below average.
Yo mama so old, she’s going to be bedridden for the rest of her life.
Yo mama so ugly, whenever she looks at a mirror, she cries a little.
Am I doing it right?
(edit for better “joke”)
Yo momma so ugly, that she walks into the haunted house and comes out with a job application
Yo momma so short, that she tried committing suicide off a bed
Yo momma so stupid, when she was about to take a drug test, she took all the drugs to study
Yo momma so hairy, bigfoot stalks her
Yo momma so fat, that she needs a wide load truck to go anywhere
Yo momma so old, that when you told her to act her age, she frikken died
Yo momma so poor, she chases after the garbage truck with a grocery list
Yo momma so skinny, she slips down the drain when she takes a shower
Yo momma so naïve, she thought golden shower was a special deal on the internet