Exceptional doesn't mean better.

Unusual strengths and unusual deficits are both exceptions to the norm.

One of the two "exceptions" is mental illness so

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.

Goal oriented self-directed behavior is what distinguishes us from apes.

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.

Life isn't deterministic.

I know literally no one who isn't quite lazy. My friends have no ADHD and they suck at time management, hate finishing projects and have a hard time focusing on boring shit.

Welcome. We call it, life.