They do sometimes in the privacy of their home to avoid staining their outfit. She could be shirtless so her “glam team” can wipe makeup fallout (when you apply a powder and dust flies into the air and settles where it shouldn’t be) off her tits without getting it on her clothes.

However, for most of the makeup application clips she wears that green tie dye shirt.
She may also just be shirtless because all the gen z girlies do their makeup shirtless and it’s #relateable, or it could be because she loves larping as kaweer and using that as an excuse to be inappropriate with others because social norms are cisheteronormative.