I think if i were to guess Its a manifestation of how she feels objectified as a trans woman, like an exotic cut of meat. Meredith can tell that she and her are alike just just from the energy exuded by her skull face. Its a manifestation of her sexual issues like Meredith's shadow, and she talks about how she feels like people could only be attracted to that part of her. I'd guess a skull was used for this because of conversations about bone structure and feeling like people can see right through her