Tattoos, piercings, and eccentric fashion are marks of free will. The most base body modification is none at all. Think of how a man would appear naturally, like an animal, with unkempt hair and long fingernails and naked. There is no choice at all in the natural appearance. Man appears as nature determines.
Next, think of man in society. He looks around him and sees how everyone else looks and for the most part dresses and grooms himself to not look any different, or at least not different enough to attract attention. Businessmen in suits, for example. In this case, man appears as society determines. In both these cases, natural and social, man does not himself necessarily choose how he appears.
It is only in the third case, that man chooses for himself how he will appear, makes his body like a painter’s canvas, and creates himself as art, such that his aesthetic appearance aligns with his metaphysical beliefs.