I would strongly advise against any type of hard plastic mirror or excessive add ons to a car seat. As someone who investigated collisions for 6 years, that stuff all becomes a projectile flying about the cabin at your vehicle speed. If baby is unhappy they'll let you know pretty quick
Agree with minimizing projectile risks where possible. There are 'suction cup' type mirrors for window attachment, but they aren't consistently secure based on reviews.
Assuming you're currently using a rear-facing child seat that relies on the seat belt and/or lower LATCH anchors, there are upper tether points (behind each headrest, underneath covers - OM19, pg 73) that are normally used for front-facing child seats. Perhaps these could be used to both tie down a mirror and control projectile risk.