User blog comment:Hawkinz340/Why So Serious?/@comment-130.156.4.250-20130930205148

I think it's referencing Heath Ledger's Joker and all his greatness - Oda must have liked it and maybe, as a honor to his "craziness" - he decided to use him in some way.

P.S. Jack Nicholson's Joker comes in a close second for me.