D.B.
This happens all the time with infants. It's completely normal. It's often on the back of the head where the most contact with the mattress happens, but it can be in other places. If you look around at other kids, you will see lots of bald patches in the back. Ignore it. Really. It will go away. I know it looks funny but there is nothing to do about it. I don't think a hat will have any effect. She will be fine.