Thats what make-up is for, isnt it ?- I didnt think Law looked too pretty in 'Road to Perdition'.
None of the other guys, who I also think would be good choices as well btw, already look like the Joker. The actor cast in the role will be made to look like him.
So what if Law is a pretty boy off-camera. It's what he looks (and acts) like on screen that matter, and I believe he could pull off a great performance as Mr.J.
But, you have your opinion, and I respect that.