It's rather creepy. The vision of yourself in the mirror tends to be distorted, and if not (in my case was not), it gets distorted with movement. As if the brain couldn't process it well, or something like that. And usually, in dreams, going into surfaces makes all your surroundings change (at least that was what happened to me, it was like teleporting).

Google for Lucid Dreaming and Mirrors, it'll get you to very interesting results.