Nicholson performance wasn't Jokerish, it was Nicholsonish. Thats right, Jack just essentially played Jack.
And he's so cool that it worked. Plus, the only other performance of the Joker that people at the time knew of was Cesar Romaro, and outside of Hackman's performance as Luthor, Nicholson's performance of a villian in a Superhero movie was it.
In other words, that performance was great for its time, but wouldn't hold much weight today.
More importantly though it that the Joker we all know an love has yet to grace the silver screen and the best intepretation so far is from TAS.
THAT IS THE STANDARD. Maybe unfairly but thats it.