Heath did a magnificent job of portraying the Joker. He was ironic and cunning, as any good Joker should be. He was also manipulative, ruthless and frightening, which are characteristics that too many action movie 'bad guys' lack. Who cares if you didn't understand his motivations. Heath made the Joker so twisted that most people were happy not to know why the Joker is the way he is and does the things he does.
In my opinion, the quality of Heath's performance warrants a nomination.