Maniac wants Mars Bar to see that blacks and whites are the same, and the plan is successful because Mars Bar has a good time.
Maniac’s idea to bring Mars Bar to Piper’s birthday party is to show both Mars Bar and the McNabs that there really is not that much difference between the blacks and the whites. In Two Mills, everything is East End and West End. Maniac wants to unite the two.
They did not go straight to the McNabs'. First they went to the Pickwells'. Maniac wanted Mars Bar to see the best the West End had to offer. (Ch. 41)
The McNabs are not expecting Maniac to bring a black kid, but Mrs. Pickwell “never batted an eye when she saw who was coming to dinner.” The dinner seems to go well, and little Dolly McNab even calls Mars Bar “Mr. Bar.” Mars Bar’s expression doesn’t change throughout the dinner.
Even if Mars wasn't letting on, Maniac could tell he was pleased to learn his fame had spread to the West. When they left, half the Pickwell kids followed them, begging Mars to perform his legendary feat of stopping traffic. (Ch. 41)
Mars Bar is able to stop traffic just like he does in the East End. This impresses the Pickwell kids and makes Maniac happy, because he feels he has accomplished what he sought out to do. He has shown Mars Bar and the West Enders that everyone can get along.
Maniac may not have accomplished anything earth-shattering, but he did help both sides take baby steps. As far as Maniac was concerned, Mars Bar was happy with the way things turned out. That was good enough for Maniac. He was trying to prove that he was friends with everyone regardless of race, and they could get to know each other too.