The joke touched a nerve because Pinkett suffers from a condition called alopecia areata, which causes hair loss.

from Times of India https://timesofindia.indiatimes.com/life-style/health-fitness/health-news/explained-what-is-alopecia-areata-the-auto-immune-disorder-that-will-smiths-wife-suffers-from/photostory/90541629.cms