If done tactfully, I imagine that it's okay to mention, but I wouldn't make it your focus. You just want to be careful not to seem like you look forward to it being pass/fail so that you don't have to try as hard or something. Also, I would try and find some other part of the school that interests you. A lot of schools do pass/fail and it might come off a little lazy if that's the main reason that you cite for your interest in their school and their program. They get a lot of applicants; you have to make your reasons stand out and seem well thought out.