We have friends who have a baby, and along with the rear facing car seat, mounted in the rear seat of the car, they use one of those little mirrors so they can watch the baby without having to crawl back into the back seat.
Recently, the mom went to a free carseat clinic put on by the local police department. After checking the car seat installation, the officer told her that baby mirrors are a safety hazard. In a crash, the mirror could detach and become projectile, hitting the baby or the other passengers of the car. Instead, he told her, "If you want to check the baby, have someone ride in back, or just pull over. If you are looking at the baby mirror, you aren't looking at the road".
This is an excellent point, and one that I did not think of before.
If you do want a baby mirror, I would consider a hard-mount, maybe by screwing the base onto the rear pillar, or bolting it down through the rear deck.