Yes, she's good too. She was like 20 when she did Fast Times at Ridgmont High, but she aged well enough to do that Harley role with the right make up and choreography.
Never heard of 'Harley Quinn' before now; way way after my time with comics; I have no idea what the character is like in the comics. but the Batman comics were always offbeat, so the actors and actresses should be too. Never saw the animated series; guess they needed a role for teenage girls to get into or something for marketing purposes.
I think Jim Carey had the best face for the Joker role, but he over-acts, and doesn't really have the right voice and body language for it, and Joaquin did a good job of it, the scenes I saw he did anyway; I never watched the whole movie of any of the Batman flicks. I just saw bits and assorted scenes when the kids were watching them on TV.