According to Wusong Road, this mug is inspired by Tibetan Mahakala ghost masks.

In Tibetan Buddhism, a Mahakala mask represents a wrathful but deeply compassionate guardian deity. Often mistaken for a "ghost" or demon, the fearsome expression is meant to protect practitioners, dispel negative energies, and destroy obstacles to enlightenment.

A crown of five skulls symbolizes the transmutation of the five negative delusions (anger, desire, ignorance, jealousy, and pride) into the five wisdoms of Buddha.