Do you sprint while looking at a mirror? You are relying on visual clues while squatting instead of listening/feeling to what your body should be doing. Thus, the mirror becomes a crutch.
When I went to a commercial gym I had to face a mirror because that is how the gym/squat rack was set up. I haven’t gone to one in almost 10 years and don’t miss it one bit. My body tells me what is going on during the lift instead of looking to see what position it is in. This proprioceptive feedback is important.