Being “too scary and misunderstood” is not a flaw. Being prone to jealousy enough to let it consume oneself is a flaw. Refusing to accept change is a flaw. Excessive pride is a flaw. Luna has exhibited all of these, and though Luna Eclipsed and her other appearances acted to move her beyond them to a point, they are still present and therefore may still be used to allow her to grow.
Luna has only had a major role in 2 episodes, yes, but Celestia’s major role has been as a plot device to summarize a lesson, not to establish a character. Never have I said that Celestia didn’t have an established personality, I have argued that she had less development as a character than her sister did. A continuously recurring character in a running show does not develop if they remain static as Celestia has. Has her role been set, yes. Has she been in many episodes and had speaking roles, yes. This does not mean that she cannot change and grow. The main characters change and grow and adapt to what they experience and learn, this is not the case with Celestia who was perfect from the start. She does not develop, she merely is.
Luna’s character is poorly displayed due to a lack of screen time, yet it is still there and she has shown growth and development as a character.
There is a difference between looking at what is canon and using it as a basis and support from which to develop an opinion, and working solely in what is shown on the screen.