What does it show about Dill as a character when he starts crying in the trial?
He has an enormous capacity for empathy and has been deeply moved by the rudeness, cruelty, and injustice he has been witnessing inside the court.
He has a weak constitution and cannot deal with life, just as he has a small body for his age and is described as being delicate and rather effeminate.
He is emotionally unstable as a result of his home life, being estranged from his birth father and having a mother who is in the process of dating/remarrying.
He is morally superior to Jem and Scout who sit through the trial straight-faced and without being moved to such emotional depths by the events' unfolding.
He understands less of what is going on than Jem and Scout do and that he is frustrated by not being able to follow the trial or predict its likely outcome.