I made Luna’s dress half armor because I wanted her to have a more guarded look, but made the rest of her outfit a large, sparkling gown to represent her beauty and regality. She holds a sickle not only because it looks like the moon, but because I imagine she doesn’t want people to get close to her emotionally.
Celestia, on the other hand, is dressed almost completely without armor save for a shoulder piece. I imagine the shoulder piece as a nod to her history of being an aggressive ruler. She is weaponless because I headcanon that many ponies find her intimidating already (hence the corona-themed fading of her gown, as it she “burns” ponies who get too close). Though outwardly, I think Celestia is a kind and nurturing figure, but on the inside I image she battles a lot of self-righteous tendencies. I’ll explore that in the future, maybe.