Your thank you letter likely adds no value at all to your application. Its just a polite reminder of more civilized times. However, having said that, it could hurt you. Why would you feel compelled to write something negative in a correspondence with the dean? Odd choice. They probably figured you were nervous when you couldn't sit still, fumbled on easy questions, and sweat profusely all over the fine Corinthian leather. That or acute opiate withdrawl. Keep it positive, even if your interview was such a disaster that your photo is almost certainly posted above the watercooler as "Worst Interview of the Month".
