It's not wrong per se but it sounds very unnatural. Tu is really not needed in this question because it's already expressed by credi (second person singular of credere).
You can still use it if you want to mark the tu but then you would put it after the verb: * cosa credi tu?* (='what is that you believe?')
Because the verb has the markings for the second person singular.
This is its conjugation (present tense):
The words in brackets, the personal pronouns, are rarely used in front of a verb in Italian, therefore I wrote them for clarity only.