This isn't specifically about Del Toro, but I really don't like how people always come up with "use make-up or prosthetics" for people who don't look an inch like the Joker to change them into the Joker.
I mean, yeah the actor who will ultimately play him might have SOME prosthetic make-up applied, just like Nicholson, but also like Nicholson, I'd like to be able to distinguish a recognisable human face behind any make-up.
I don't want to be staring at a malleable face-mask. It should be the actor playing him, who is able to use the full range of facial expressions. No huge chin, cheek stuffing and disproportionate nose jobs. Very subtle enhancements, yes. But the actor should look like him to some extent before any make-up comes on.
As for Benicio Del Toro, there is nothing wrong with that choice, but he's not a favorite of mine. I think other people are better suited, definitely in looks and character-wise.