Ok, as this foals teeth come in, and the baby grows, PLEASE have her back teeth and all monitored for hooks that will develope BECAUSE of the current underbite. The hooks will wear and get worse and prevent the bite from going back on normally if it's going to, because they will hold the jaw where it is, or worse.
Make sure this baby gets good dental care... and I would have the foal checked as it grows to see what is happening in there. Many times, hooks can develope in the back, where you cannot see, and you have no idea what is going on in there.
The underbite could be caused by a variety of reasons- not enough room inside, or another problem. A good equine dentist can possibly take a look in a few weeks and check her out and let you know. Dont let her grow for a year or so and then decide to have her checked. She needs every advantage she can get to straighten out as she grows.
And yes, they CAN come back on, but sometimes just need the hooks kept off to keep the bite good and flat. I bought a young mare that had a terrible bite, and with just regular dental care (that didnt start til about age 4 by the way) to keep her floated, her bite DID come back on by itself!!