If they weren't in the middle of a life or death clusterfuck battle, Joker would take the time to laugh at that. Unfortunately, no time, and it's clear Strange won't be convinced- at least not now. For obvious reasons. So Joker takes off a shoe and chucks it at the back of his head instead.

"Then don't act so surprised!" he calls before he darts through the air to take care of other business. There's an amulet to defend.

And then lose.