I don't remember. I am almost sure she was Margaret Angélica Mendoza before Chichen Itzá and I knew somebody (I did not remember who, so Lara, it could have been the gatekeeper) took care of the legal part, but Harry "was dead" so I was never sure if the bothered given her his name, especially knowing all the enemies Dresden (and Eb) had. So, I just had the idea that perhaps the Carpenters have been appointed the legal guardians of Margaret Mendoza.
I mean, Susan hide her in the system so their enemies did not find her. Calling her Margaret Dresden would have been so stupid like trying to hide the son of Anakin Skywalker in his birth planet and keeping his name.