Look a Little Deeper
If you just look at the surface of a character you oftentimes miss some of the most important aspects they have to offer. If you look deeper you'll find the true person.

Luna is quite the nag when it comes to keeping Usagi in line, but with Usagi is seems to be necessary. Luna genuinely cares about Usagi and when it comes right down to it she believes in her abilities which is why she would choose her to lead the girls. She watches proudly as Usagi matures over time and tries to do her best with Chibi Usa as well. In the S movie Luna shows how she can be when she is in love. Luna seems to be almost embarrassed by romance but when she is confronted by it, it makes her heart of gold shine through. As a cat or human she has shown that she can be noble and good.

The Ups and Downs
Every person has their good points and their bad points. That goes for animated characters as well so I have make a little list to go over some of these points.

Character Pros: She tries her best to keep Usagi in line. She has lots of useful items to give the soldiers as they further evolve.

Character Cons: She appears to be mean when she is strict. She nags a lot.