How are ditzy characters classified? 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.