Body: You could have done the body a bit better... She is a supposed to be thin, correct? In this drawing she looks more chubby than thin and I can bet you anything Joker did not fatten her up THAT fast. Besides that you did her arms pretty well, they look strong. Strong enough to carry that big hammer, no less. Her head looks alright, though I feel her mouth and nose could have been better shaped but for how most faces look you did a pretty good job there. Legs look good.
Clothing: Well there's really nothing that bad I can say about them besides that the sleaves on her white t-shirt aren't exactly looking that matching, one is loose around her shoulder while the other is tight on the other. Now I am not a great artist myself but isn't that not a supposed to happen?
The rest: Can't help but notice that the hammer Harley is holding seems to not be exactly straight as it could be, but that is the tough point when it comes to digital drawing. As for the blood falling from the hammer, it looks pretty good but I think it could be a little darker.Nothing bad can be sad about the diamonds, they look good. The arm, that shows that Harley killed a man, should be noted that it somewhat overlaps with the black diamond but it's nothing too bad so I don't mind much.