The Ending (spoilers)
If you were to ask me halfway through what this movie was about, I would have said that it was Jimmy's goal to restore his brother's health benefits through swaying public opinion by spoiling the mayor's re-election. However, neither of these happens before the movie ends. The final stunt seems to be more about "Can he do it?" than those two issues.
So by the end, he has completed his final stunt, but we don't know if it will do his brother any good (other than being proud of the kid), or if the city's politics will change. Which is strange, because I don't think the film's message is that graffiti is useless to effect social or political change.
Do fans of the movie just assume that their problems got resolved? I'm curious.