Well done! But, there are things I really like and some I think are a bit off.
+ :
1.The hairs are stunning! The shading in them is brilliant
2.The shading and cross hatching overall is really good.
3.Your lines are strong
- :
The main problem is, I think, your proportions. Now, you may have chosen to do some of the things i'll talk about for stylistic reason. But I think in the overall drawing they are a bit off with the rest.
1. The neck seems a bit too large and long. His jaw is really long so if you draw the neck too much after the chin(like you would do with a normal drawing) this gives a weird impression. Yes, the joker's face is not proportioned like a real person, although the rest of his physical attributes are more human-like. And it seems a bit large also, which gives the impression that his head is a bit small.
2. The upper lips is way too large for a smile. When people smile, the upper lips becomes really small, almost disappearing. By the way on a man, both lips are about the same size or almost.
3. The eyes. I understand the facial expression you wanted to give him, but they seem too cartoonish for the rest of the drawing that is really comic book-ish. The upper eyelids are missing and it gives a weird look, almost like the eyes aren't stuck in the face.
Overall, the drawing show that you clearly have talent!
Keep up the good work!