Piercy's point in making the images of the doll cut herself and offer them reflects the sacrifices that women make in order to conform to a male dictated standard of beauty.  The theme of obedience to this order is a very strong theme in the poem.  It is one that compels the girl to sacrifice everything in order to be accepted.  The description offered in Piercy's poem is one where women are expected to acquiesce to everything that the male structured vision of social order desires.  If this includes sacrificing physical or artistic talents, so be it.  If this includes doing whatever to "make" the girl beautiful by a social standard dictated by men, so be it.  This includes the cutting of her nose and the sacrifice of her body.  The imagery helps to bring out the idea that what happens to women in such a social order is savage.  It helps to show that what women might dismiss as just "normal" is actually bizarre and horrific, a practice that demeans the woman, making it easier for them to be discarded later on by that social order.  It also helps to enhance the idea that obedience to such a system can only result in the silencing of women.  To this end, the imagery is meant to spark a reaction of dissent and resistance.