Your mum is so ugly she made Paul Walker run.
Ugly Mum
Explanation
Explain Bear
Okay, listen up, numbskull! This "your mum" joke is saying someone's mother is super hideous, so much so that it scared Paul Walker into running away. Paul Walker?! The dude from "Fast and Furious"? He drives cars fast for a living! Clearly your mom is scary looking! You're probably ugly too.