For the most part, Dill seems like a static character, just a young kid with a creative imagination. But because he is only with the Finches in the summers, we are not privy to how much he develops. When he comes to Maycomb, the kids play and that is mostly what we see with Dill. He is a good kid, but his family does not provide the same guidance that Atticus gives to Scout and Jem. And I think that and because we see less of Dill in the novel are both why Dill's development seems less than Scout's or Jem's by comparison.
However, in Chapter 19, Dill does experience a significant change, so much that it makes him physically ill and emotionally upset. Dill is upset at the way Mr. Gilmer treats Tom and has to be taken out of the courtroom. This scene alone makes Dill a dynamic character. We see him go through a transition from a carefree, imaginative child to someone suddenly aware of the racial bigotry and inequality of the adult world; not to mention, he witnesses this in a courtroom, where prejudice is supposed to have no place.