The upper and lower jaws are parallel, making it diffucult to imagine how this mouth can close, you should angle them. It also looks kind of unrealistic because the mouth hole ist just pain dark. With just a subtle hint of the other teeth and a tongue, it would look better.
And as a last, but more difficult point to fix: To get the teeth in and out of the vitim, the teeth are normally bended in a way that they follow concentrical circles around the jaw hinge. Your teeth are much too croocked (look at the first upper one - its bending center is inside the tooth space, but as mentioned before, the hinge is actually very far away.)