How are ditzy/airhead characters classified in anime/manga? For example, while Lucky Star's Kagami is a tsundere, her airhead sister, Tsukasa, is a ________?
You are probably looking for the term dojikko for Tsukasa:
Dojikko is Japanese for clumsy with the feminine suffix "-ko" added on the end. The attraction is not limited to Japan, the concept of a Cute Clumsy Girl has worldwide appeal, mainly because her clumsiness makes her more approachable. The main difference is that in the West a Cute Clumsy Girl is probably supposed to make the audience chuckle, while Dojikkos tend towards making them say, "Awwww! She's so cute!"
Source: TVTropes article on dojikko
Also, Kagami is not really a tsundere, unless you think she is in love with Konata, which some fans really do seem to support. She should probably be classified as the tsukkomi (straight man) in manzai, where Konata is usually the boke:
Called manzai in Japanese, this is a kind of Straight Man and Wise Guy duo, but it's also the interaction between two characters who constantly play off each other. A boke's job is to set up the gag by telling a story or explaining a fact which is obviously false (making him an idiot) or flawed (making him sneaky). The tsukkomi is, roughly, the Straight Man who has to correct him, often physically.
Source: TVTropes article on manzai
Edit: Since dojikko didn't seem to be a proper answer, I looked up more character types on Japanese websites and think I found exactly what you're looking for. It actually fits in really well with my explanation of manzai above. My new answer is that Tsukasa is a tennen boke (natural boke, also translated as natural airhead):
A tennen boke is an eccentric person who has exceptional talent in being a natural boke. Even though a person can be called a tennen boke, there is no such term as tennen tsukkomi. In manzai, the boke deliberately entices both the tsukkomi and the audience's laughter by executing a calculated line. However, the tennen boke describes someone who naturally (without any preparation or purpose) says boke-like things.
It can also be called simply "tennen" or "tennen kyara" (tennen character).
Source: Translated from the Japanese Wikipedia article on tennen boke
So basically, a boke is someone who says dumb things on purpose in order to anger the tsukkomi and make other people laugh. However, a tennen boke says the same dumb things without realizing they are actually dumb.
The Ditz is a character whose defining characteristic is profound stupidity. Female ditzes tend to be sweet and naive, while male ditzes tend to be oafish but lovable. The Ditz is written to appear unintentionally funny. In drama series, he or she provides comic relief.
I think Tsukasa's character doesn't fall into any of the deredere classifications because first of all,
lovestruck and she doesn't have any love interest in the anime.