Compiled from the Twitter contest held by @nerdist.
JanetVarney: Yo momma so fat the Sorting Hat put her in all four houses
chris8675309: Yo mamma so stupid, she thinks Sirius Black is the soul station on satellite radio
sarazafar: Yo mama so fat she tried to eat Cornelius Fudge
dino_rider: Yo mama’s so ugly the Dementor’s Kiss was swapped out for a hearty handshake and a promise to give her a call sometime.
bradheintz: Yo mama so nasty, Dobby wouldn’t take her sock
Burnaway: Yo momma so skanky, even her patronus got knocked up
chompychomp: Yo mama so stupid, she drowned in a pensieve
Dick_M: yo momma so dumb she thought that she could talk to snakes if she put parsley on her tongue
dtejano: Yo mama’s so masculine, she makes Dumbledore go, “DAAAYYYUMM!”
habcous: yo mamma so nasty, every pair of her panties has the Dark Mark
HakSolo: your mom is so fat if she confronted a boggart it would morph into a treadmill
dantelfer: Yo mamma so fat, she has to enchant her panties into a portkey to get out of a chair
dirk_funk: Yo mama so fat it takes two boggarts to shape-shift into her
MarioEGarcia: yo mama so poor she went to Honeydukes and put a chocolate frog on layaway
melgotserved: Yo momma’s so fat she got stuck in the Floo Network
melgotserved: Yo momma’s so fat her wand is a Slim Jim
MikeDrucker: Yo’ Momma so fat she joined the Death Eaters ’cause she was hungry
MsJG: Yo Momma’s so fugly that even Voldemort won’t speak her name
paulscheer: Yo Mamma is so fat that even the Dementors can’t suck out her soul in one sitting
phirm: Yo Mama so nasty, everybody call her “She-Who-Must-Not-Be-Naked”
Taborifica: Yo momma so fat even her Quidditch robes have stretch marks
MarioEGarcia Yo momma's so fat the sorting hat assigned her to the House of Pancakes
cvgurau: Your momma so dumb, they put the Sorting Hat on her head and all it heard was an echo.
chris8675309: Your momma so fat, she looked in the mirror of Erised and saw a ham
DavidAkers: Yo momma so old, there ain't an Age Line that can stop her.
elchupahueso: your family's so poor that you make the weaseleys look like the Malfoy's!
girl_noir: Yo mamma so fat, she splinched herself an' nobody noticed.
HakSolo: Your mom is so ugly when she walked into Gringotts Wizarding Bank, they gave her a job application.
hollibo83: yo mama so ugly when she was born the doctor screamed "riddikulus"
MarioEGarcia: Yo mama so old, she used to babysit Dumbledore
DavidAkers: Yo momma so ugly, no one can tell when she's an Animagus.
R-Silent: Yo momma so fat her patronus is a cake