A lot of Luna's appeal to the audience is that she doesn't have the total confidence that Celestia does. Celestia's public presence, and even her private one to an extent, is perfect, infallible, unassailable, invulnerable, eternally wise.Luna, while more comfortable with ruling a country than, say, Twilight (who also has a very similar vulnerability appeal), is unsure of herself to a degree, and deliberately visually presented as being smaller and younger than Celestia. She's also more out of touch with modern society - another flaw that Celestia doesn't have - and sometimes behaves inappropriately (the Canterlot Royal Voice, amongst other things). She's scrambling to be worthy of the role she's been handed, much the same as Twilight, although with the variant that it's a role she once fit into perfectly but no longer does as well because it's changed out from under her. (On the other side of the coin, Twilight is scrambling to fit into the role of Princess for the first time.)Effectively, Celestia is most often presented as being almost entirely subsumed by her role. Which is not surprising - for a thousand years she's beenPrincess; barely anypony knew the true personality behind the crown. Luna, on the other hand, is seen first as Nightmare Moon, and then as the tiny blue almost-foal when she's separated from it, and then we're given glimpses of her interacting far more one-on-one with the populace (both in person and while dreamwalking).Luna's more relatable because her problems are personal (speech patterns, fitting in, having to learn the current state of Equestrian law/relations/society/technology/culture, while Celestia's problems tend to be presented as problems of the position - wars, diplomacy, ambassadors, monsters attacking Equestria in general, having to walk new alicorns through their ascendancies...In short, Luna is a, Celestia is a