Can I sue a dermatologist for the cost of my visits and medication if her prescribed acne treatments left me worse off than before?

It depends on whether:
1) It was appropriate to prescribe or do the treatments she did, or if they were inappropriate for your condition or type/severity of acne; if a reasonable dermatologist would not have done these treatments or prescribed this medication, it may well have been malpractice.
2) Even if the treatments were, in theory, appropriate, if this doctor administered them wrong, or gave you the wrong dose of medicine, or missed obvious warning signs of a problem, that may be malpractice.
3) Even the treatment/medicine and the way it was administered was correct, if you suffered from known side effects but the doctor failed to warn you of the possible side effects and get your informed consent, that may have been malpractice.
On the other hand: if the treatment and medicine was appropriate; it was adminstered correctly; and you were warned of known side effects, then there is no malpractice, because in this case, the doctor did everything right. Malpractice occurs not every time a patient has a bad outcome, becaue the law accepts that medicine is not perfect; rather, malpractice occurs when the doctor does something wrong or careless.
Note that to sue a doctor, you'll need a medical expert who can testify that the other doctor was in the wrong and caused you harm. A good first step is to consult with another dermatologist or other doctor and see what he/she says about the cause of your problems. If another physician feels that you suffered due to the treatment, then you may have a malpractice case.