Squatting in front of a mirror is a really bad idea. Many weight rooms have mirrors on all the walls, making it impossible to squat without a mirror there, within eyesight, giving you its bad feedback. A mirror is a bad tool because it provides information about only one plane, the frontal, and depth cannot be judged by looking in the mirror from the front. Some obliqueness of angle is required to see the relationship between patella and hip crease, but a mirror set at an oblique angle would produce a twisting of the neck. Cervical rotation under a heavy bar is just as bad an idea as cervical hyperextension under a heavy bar. But the best reason not to use a mirror in front of any multijoint exercise is that you should be developing kinesthetic sense of movement by paying attention to all the sensory input provided by proprioception, rather than focusing merely on visual input from a mirror. "Learn to feel it, not just see it," is excellent advice.