@CrashGordon
You know… Some could argue that the Moon could fall under "Light is Good" and "Light is not Good" if you note the fact that the Moon is the biggest source of Light at night.
And if you look at Luna vs Nightmare Moon… It's kind of like comparing a Full Moon to a New Moon or a Lunar Eclipse.
As for personality traits, In Magic the Gathering terms, Luna is Red/Blue, and Nightmare Moon is Blue/Black. As in, Luna is about inspiration through the intersection of emotion and knowledge, while Nightmare Moon is trickery through the intersection of knowledge and self interest.
Speaking of Magic… There is one example of an Angel of Pure Light in that game being associated with the Moon to some extent, and even being the source of redemption for a race of werewolves, and was the protector of humanity against the forces of darkness that ruled the plane… But that's neither here nor there.
Getting away from Magic… It's unfair to really call Darkness itself, or the Night, or even the Moon "Dark" per say. As being "Dark" actually implies a lot of things that don't strictly speaking have anything to do with being associated with the night.
For example… A Vampire is dark not because they can't go out in the day, but because they feed on humans. Zombies are dark regardless of the fact they can go out in the sun just fine, because they subvert the "natural order" and exist to prey on the living. Bayonetta's takes on Angels, despite the fact they are beings of Light are still "Dark" in that they are terrifying monstrosities that seek to destroy the world and have no concern for humans at all, and basically use them as pawns.
You want to know what qualifies as "dark" in MLP? The Timberwolves, Tirrek, Sombra, Windigoes, Cockatrices, that Chimera, Trixie with the amulet, and Queen Chrysalis.
What's not Dark… Is Luna. And you want to know why?
Because all of those that class as "Dark" are in some sense or another a force of amorality and self interest, as well as perversions of life itself.
As such… In order for the "Dark is not Evil" theme to be applied correctly as a tittle, it must be in terms of a selfish hero who is not saving the world for the sake of others, but because they themselves want to keep the world around to they themselves can benefit from it.
And Luna herself? She's none of that… Not since she got over her little grudge. So it's impossible for her to fit the true themes of "Dark is not Evil", as she's way too kind and helpful when she's herself to fit any kind of dark themes.
You know which "Good" character DOES fit "Dark is not Evil" in MLP?
FREAKING SPIKE!
So the lesson to be learned here is… That you should only complain about "Dark is not Evil" when you're talking about in terms of "GrimDark vs LightFluff".
And Luna is very LightFluff!
P.S. Anyone who finds literal Darkness/Night/Moon being good annoying, probably hasn't gotten rid of their childhood fear of the dark yet.