Any insult that fits the following pattern:

Yo momma is so [derogator attribute], she ....

For instance:
  • Yo momma is so poor she can't afford to pay attention.
  • Yo momma is so nasty she made Right Guard turn left.
  • Yo momma is so ugly, when she goes into a bank they turn the security cameras off.
  • Yo momma is so dumb it takes her 2 hours to watch 60 Minutes.
The genre is so big that Google even has a single page dedicated to sites with Yo Momma jokes (Recreation > Humor > Jokes > Yo Momma Jokes).