I believe that Atticus is showing his hopeful side when he claims that the Ku Klux Klan has disappeared from Maycomb in Chapter 15 of To Kill a Mockingbird. Jem has just questioned Atticus about his safety while he was defending Tom Robinson in front of the jail, and Atticus is trying to calm Jem's worries. Atticus, of course, is wrong about the Klan: They were (and are) still alive and well. It may be that the Klan was less visible in Maycomb during the past few years--Atticus tells Jem that they were much more evident around 1920. It may also be that the Klan (in Alabama, anyway) were just beginning to turn their interests more directly toward actions against African-Americans than other minorities (Atticus mentioned Sam Levy--presumably a Jew--shaming the KKK away).
I suppose that you can say that this makes Atticus naive about the level of racism that persisted in Maycomb in his time. However, I do not think it does. In my opinion, it shows that he is realistic.
The reason I say this is that the KKK really had not been very active in the South for quite some time by 1935. The "need" for the KKK had gone when Reconstruction ended and white people were on top again in the South. I mean, when the judicial system kills Tom Robinson for you, what do you need the KKK for?
So if Atticus had said there was no racism, I'd say he was an idiot. But he knows there's racism. He just says the KKK isn't that important.
Now, give the context where he says this, I think he's just brave. He's trying to reassure his family -- tell them that the mob is no big deal. That's brave.