In the first example,
No person could have been more nationalistic.
there is an implied subject of comparison, probably a particular person who has already been established as the subject of discussion. A more complete statement would be,
No person could have been more nationalistic than Joe.
The literal meaning is, "it is not possible for a person to be more nationalistic [than Joe]", or "Joe is as nationalistic as it is possible for a person to be."
Such phrases are, of course, often used with an element of rhetorical exaggeration or hyperbole, so it's possible the speaker doesn't actually believe the statement in its literal sense.
I couldn't have been better.
Here the implied comparison with other states of being you might have had at some moment. A more complete statement would be,
I couldn't have been better than I was [at that moment].
There's no "supposed person" here because what you are implicitly comparing is not between you and other people, but between how you were and how you could have been.
This can possibly be changed through context:
Joe was good at chess. I couldn't have been better (even if I practiced daily).