Me: Hey, say I am ugly for a billion pounds.
Them: You're ugly.
Me: Sorry, I am not a mirror.
Alright, listen up, buttercup. The joke is that the person wanted someone to say they were ugly for a billion pounds, but then they clapped back by saying they're not a mirror. Because only a mirror would reflect ugliness. I bet you still use training wheels on your bike, don't you?