Juan Diego was a illiterate Indian. He was not a doctor of reformed theology, or any other kind of theology. Sometimes God reaches us down where we are, even though you don't approve.
I have yet to see Mary looking like a short dark Middle Eastern as she most likely was; rather than the African, Oriental or anything else. It is understandable when people paint pictures of Mary that she appears like this because it is an artist's interpretation. However, when people say that Mary stamped her picture on their T-shirt, then one would hope that it would be an exact likeness of her. If you compare Juan's pictures among all the other pictures from the few who have similar stories, then you would think the likeness would be the same. She wouldn't look like she just came off a catechism card.
It isn't a matter of what WE believe. It is a matter of what scripture tells us.