I’m sympathetic to 2DT’s take. I always thought Azusa was someone who had to be liken to in some way to a previous experience. The stereotypical saccharine-filled neko-mimi sweets-lover is not why she won Saimoe last year, nor is it why Azusa is probably my favorite K-ON girl (tho I think Yui is still the best, whatever that means).
To put it in the right context, 2DT saw the light because he realized that Azusa is actually based on some real-life notion of ideals. In the comments he explicitly stated this connection (emphasis and added link mine).
I wasn’t so much a fan of her myself… until I realized that all the students I have who are like her, I love!  Such a sweet, good child. I understand her classic moe pull.
I’m not going to talk much about that classic moe thing (by the way, classic moe is better a term than old moe, because there’s, well, actual old moe involving old people). It is different than the somewhat more mature, hime-cut Yamato Nadeshiko moe (Really? You’d think a Yamato Nadeshiko is naturally attractive?) that is better characterized by Mio, so even “classic” is not exactly a great moniker. Furthermore, moe is so dead, I would rather not talk about it if I could avoid it.
What I will share, however, is my previously-mentioned (well, it was over a year ago) note on how I like K-ON because I see that band dynamics play out in my own experience. In short, Mio reminds me of this guy I know. He is adorable, musically talented and everyone likes him, just like Mio is among her friends. Both of them had a fan club. Well, YMMV, as it is with anything personal.
People are multi-motivational and complex things. It takes time to get to know them, and it is is tough to do fiction where part of the charm comes from mapping characters to actual people in a typical yet lifelike way. The good-girl appeal, however, is always going to be a draw to people who like them. I think it channels some sense of righteousness that many of us have within us, a sense of right and wrong, or good and evil. And going by that measure, that applies to most. I just want to separate out the two different kinds of good girls.
So the story goes, to begin with a scene from Conan the Barbarian (lol thanks JP):
Mongol General: Hao! Dai ye! We won again! This is good, but what is best in life?
Mongol: The open steppe, fleet horse, falcons at your wrist, and the wind in your hair.
Mongol General: Wrong! Conan! What is best in life?
Conan: To crush your enemies, see them driven before you, and to hear the lamentation of their women.
Mongol General: That is good! That is good.
Thanks to a certain parody subber, I have long since fallen with Simoun–more importantly, Mamiina. Simoun has a large cast that comes together for some pretty delicious drama. One of these is exactly the very heart-warming, turn-around good girl that newfags may find familiar in the character Kyouko Sakura from Madoka Magica, if Simoun is unfamiliar to you. Just like Kyouko, Mamiina is not a model citizen. In fact there’s nothing to her that makes me want to stick a pair of cat ears on her head. It’s the difference between a stray cat and a house cat, I guess. But it’s just that Mamiina screams loudly, through her actions, that she is indeed the very sweet, good girl you find in Studio Ghibli’s adventures, even if she is at times antagonistic.
The same idea plays out along the lines of all these Azusa-type moe characters. This is where 2DT’s observation comes into play, but only to a point. Because while we have our Kikis and Sheetas, there’s the Sans and, arguably, Nausicaas in the Ghibli lineup. I’m going to say that there are two sides to the Good Girl archetype, and the difference between the two is a philosophical divide in which only some of us can reconcile.
Back to Mamiina. She is a good girl because she jumps this gap between antagonism and protagonism in order to demonstrates her inner qualities, and she does the Right Thing at the Right Time. It leaves an emotional crater, a desired dramatic impact, especially when framed in the context of her intercharacter relationships. But at the same time, I can imagine a Mamiina-type character would be the nemesis or rival to an Azusa-type character (it probably is the plot to some battle show, wouldn’t surprise me). Without the external stimulus that put these characters through the crucible of tragedy or suffering, maybe the difference between the two types of good girl archetype lies within the answers between the random Mongol and Conan. Conan was just some normal barbarian at first, after all.
Another hypothetical may demonstrate better. Remember the YuiAzu episode? Let’s say if we replace Azusa with the antagonist version of Azusa, what would instead happen is that rather being a type A Good Girl, the antagonist Azusa would not volunteer to help Yui with the performance, yet later on do so anyways for some plot-specific reason. It seems like a typical gap moe tsundere type thing, but that is not directly relevant to a demonstration of character. You can be good or bad and still possess this duality.
I suppose that’s all part of the specification of being good; there is a strictness within Azusa’s tenderness, or the flip side of the coin, a tenderness in Mamiina/San’s intensity. It turns some of them evil, some of them slaved to passionate antagonists, some to reason and some to strange plot devices about clones. I’m not sure how much I can stretch this, but all I’m trying to say is that are Good Girls among us, and there are many. And like girls generally, they have a plurality of face!