Also inappropriate for a professional interpreter. You have to have your poker face on at all times. You are interjecting your opinion with an evil glare just as if you say something "out of role," or if you choose not to interpret something. None of these are okay. And no, this isn't easy, but it's part of being an interpreter; you cannot appear to judge any participant or take sides, no matter what you think privately.
Also by antagonizing the AP, you may be cementing in this person's mind that interpreters -- and by association, deaf people -- are unpleasant to work with.