I am not entirely sure what you are asking, but I'll give it a shot, first in relation to Dill, and then in relation to the verdict. In either case, I think Atticus is absolutely acting "within character."
As for Dill, Atticus treats him with the same respect he treats any human being, no matter race, color, creed or indeed, age.
If we are speaking of Atticus's rather stoic response to the unjust verdict, this is also within character but in a somewhat different way. Atticus always maintained a consistent air of control. He knows from long and difficult experience that grandstanding and shouting will not help. The only clue we have to his actual state of mind occurs in the end of Chapter 21:
...(Scout) I saw Atticus pushing papers from the table into his briefcase. He snapped it shut, went to the court reporter and said something, nodded to Mr. Gilmer, and then when to Tom Robinson and whispered something to him. Atticus put his hand on Tom's shoulder as he whispered...Then he left the courtroom, but not by his usual exit. He must have wanted to go home the short way..