I haven't observed any correlation between being nervous and underperforming in answering technical questions. How do people get through math exams in college? If anything, I'd expect answers to non-technical questions to be far more influenced by the artificiality and unfamiliarity of the interview situation, being judged by a stranger, high-stakes, and everything.
I actually got a lower GPA in college than I could have because of test anxiety issues in college, and I've since learned that it is hardly a rare condition.
