Sakaki, Parody, and the Somber Girl
You know her. You've seen her before, in just about every anime or manga series that you've ever encountered. She's the silent girl. The one who never speaks, who walks around with an unchanging, deadpan expression on her face. When she does open her mouth, her voice is low, breathy, and monotone. She is an enigma. Behind the expressionless face and somber eyes lies an emotional depth, a weighty tragedy, a sorrowfully wrought persona that we, the viewers, will never actually see surface.
You've seen her before. She is Ayanami Rei, she is Hoshino Ruri, she is R. Dorothy Wayneright. She is... Sakaki-san?!
Not quite.
I remember very clearly a conversation I had with a friend, my freshman year of college, late one evening, after an anime club showing. We had just suffered through yet another two episodes of Dual!, and the topic of conversation happened to turn to D, a certain dead-eyed, pale-faced, monotone-voiced female character from the series. My friend suddenly blurted out, "I hate those anime girls who are, like, so silent, but they're supposed to be, you know, all deep and stuff."
Amen to that. If there's one stereotypical character type that's been done to death in anime, it's that girl. And by now I hope you have a clear idea of who I'm talking about. She is Yuumura Kirika from Noir, she is Miharu from Gasaraki. And, at least superficially, it looks like Sakaki could be the perfect candidate to fill out Azumanga Daioh's obligatory Somber Girl niche.
But the great thing about Sakaki is that she doesn't fulfill the Somber Girl character type. In fact, she takes the entire clichéd construct of the Somber Girl and turns it on its head. How? By not being "deep."
The defining characteristic of the Somber Girl is that she's supposed to be, as my friend put it, "all deep and stuff." Beneath that pale mask of face, beneath those dully slanted eyes, lies some sort of secret pain, some sort of consuming tragedy. The Somber Girl is an enigma because we, the viewers, just know that there has to be more going on in her mind, and in her heart, than she's letting on. And that usually turns out to be the case.
Sakaki, however, is NOT "all deep and stuff." In fact, when you strip away her expressionless face, low, quite voice, and deadpan expression, you're left with... warm, pink, cuddly fluff. The most that seems to be going on inside Sakaki's head at any given moment is probably a pink-tinted daydream about petting cats, or at the deepest, about Chiyo's "Father" playing baseball. Sakaki's greatest tragedy is not that nobody loves her, not that she has to pilot a giant robot to find meaning in life, not that she has to assassinate others in order to make a living. No, Sakaki's greatest tragedy is that she can't find a cat to pet that doesn't want to bite her fingers first.
Since Azumanga Daioh deliberately (and gleefully) derives a lot of its humor from making fun of stereotypical anime characters and situations, I personally tend to think of Sakaki as a parody of the Somber Girl stereotype. Instead of a deep tragedy, Sakaki's silent, awkward veneer masks nothing more than a fluffy, childish mind.
So, that's basically the conclusion. Sakaki is a parody of the Somber Girl, who approaches her single-minded quests to pet cats and pursue cute things with the same stalwart conviction that propels Rei to pilot her giant mech, and that propels Kirika to search for her past. The only thing that could make Sakaki fit the Somber Girl stereotype any more would be if she actually turned out to have some sort of tragic secret in her past. For this reason, I wouldn't be surprised if someday a sequel to Azumanga Daioh was produced, in which it would be revealed that Kimura-sensei is Sakaki's father. ;)

You know her. You've seen her before, in just about every anime or manga series that you've ever encountered. She's the silent girl. The one who never speaks, who walks around with an unchanging, deadpan expression on her face. When she does open her mouth, her voice is low, breathy, and monotone. She is an enigma. Behind the expressionless face and somber eyes lies an emotional depth, a weighty tragedy, a sorrowfully wrought persona that we, the viewers, will never actually see surface.