Perhaps the mask is just a fashion choice or it might be related to a specific story or event in the BT21 storyline that gives a reason for Mang to wear it.
There were several reasons why Yang Liping always wore a hat. First of all, she was a Bai woman. Wearing a hat might be a custom in her hometown. The hat was also a part of the Bai women's clothing, which helped to show her ethnic characteristics and stage effects. Secondly, Yang Liping had been practicing dancing for a long time. In order to present the perfect stage effect, she often practiced repeatedly, causing veins and wrinkles to appear on her face. In order to cover these flaws, she chose to wear a hat. Lastly, Yang Liping also had her own personal preferences. She liked to wear ethnic clothing, and hats were one of her personal habits. In general, wearing a hat was Yang Liping's choice to show her ethnic characteristics, cover her facial flaws, and satisfy her personal preferences.