There are other axes by which to assess humans than "acheivement". There are even other axes for value! Someone can be "exceptional" along any of them.
Underachieving is just inability to deploy your efforts effectively, or even inability to muster an effort in the worst cases. That translates to every domain and aspect of human life from scholastic to social to friendships etc etc etc.
Value is not entirely subjective.
Welcome. We call it, life.