Most people use this phrase to roast or make fun of somebody because if you care about your mom you would be insulted if they started a roast with "Yo Mama"
Guy 1: "Yo Mama so dumb, she bought a solar powered flashlight!"
Guy 2: *in tears* "Wow thanks dude..."
Used as a form of burn. Originally from Ireland with the term u nasty. Now a days the pop culture uses this term to did on each others mama's. Go kill your self for disrespecting the mama's around the world.
"Yo mama so ugly, one direction went in the other direction"
"Yo mama so ugly, she looks like u. The one reading this sentence. Yeah, YOU. Thanks."
That's me, yo mama you've all been offending.
guy 1: yo mama so fat, when she walked past the tv I missed all 20 seasons of the Simpsons.
yo mama: hey you ugly bitch, that's me your talking about.