21 y/o f, what does it mean if the urinary bladder walls are not well-distended?

The comment is just an observation by the interpreter that there is not much pee in the bladder. It has no negative implication. They like to have a full or near full bladder on some studies as it allows a better look at the bladder lining in the full state.

This is a very vague report and has to be interpreted in context with the reason for the test. We need that and the full report before comment