In addition to the very good answers given by people above, if you believe in private revelations such as apparitions of Mary, she has appeared looking all kinds of different ways. She generally looks like she could be the mother of whoever she is appearing to, because she is mother of us all. Therefore, when she appeared in the approved apparition Our Lady of Good Help in Wisconsin, to a Belgian immigrant girl, she looked like a fair-skinned blonde. When she appeared in the approved apparition Our Lady of Guadalupe in Mexico, to an indigenous man who had converted to Catholicism, she appeared as a dark skinned, dark haired mestiza, a woman of mixed European and indigenous race, symbolizing the blending of the indigenous culture with the Catholicism brought by the Spanish.
Therefore, the appearance of Mary is often symbolic rather than an attempt to show Mary exactly how she looked when she was living on earth.
