This is for sure a paint…
This is for sure a paint defect. Mirror housings are plastic. There’s no removing dents from them. It would have just cracked if it were damaged. Applying too much heat to paint wouldn’t melt it either. If anything it would blister and crack if it’s a QUALITY paint job. The only way this should ever be possible is applying new paint too heavy, in which again, is not a quality paint job.