If I have a bunch of neighbors who want me to paint my house blue because the people across the street hate that color and they want to piss them off, I might not want to do it because I have no beef with the people across the street, and no desire to help my neighbors irritate them. However, if I want to paint it blue for my own reasons, I'm going to do it regardless of anybody's opinion on the matter. Is it a victory for my asshole neighbors? Not if I based my actions on my own desires, and I'm satisfied with the results. Are they going to be happy about my decision? Probably. Might they think I did it based on their advice? Sure. Either way, it's no concern of mine, because I don't let others' whims dictate what fulfills me.
Well see, the difference between your example and the film is that, in the film, there's a third party involved: the painter. If the prank in the film didn't involve physically injuring another person or ruining the man's work (and quite possibly his job), then I'd be okay with the film pulling a prank at the end. But by pulling the prank, Ichiro is harming the painter.
Look at this from the painter's perspective: Some kid he doesn't know honks the horn of his motorcycle, which causes him to fall off his ladder, which could have hurt him, causing him to spill his paint cans, which could very easily be worth at least a day's pay (if not more), and everything that he had been working on that day has been ruined. I don't think he sees honking the horn as a prank. He probably sees Ichiro as nothing more than a stupid and messed-up kid with lots of problems and bad parents.
One thing I keep leaving out of my argument is that, even if what Ichiro does is nothing more than a prank, therefore it's okay that it goes against the moral of film, Ichiro is still harming another individual, which is exactly what the bullies were doing to him. In which case, Ichiro is now no better the bullies. Our hero, ladies and gentlemen!
I can see your point about Ichiro pulling a "different" prank to avoid this overlap, making the dramatic arc flow a little less ambiguously, but I reject out of hand the notion that his prank as-filmed represents a victory for the bullies. They might enjoy watching it, but Ichiro shouldn't give two-shits about it. Does this make Ichiro a bully in his own right? Not in my mind, but lets say for the sake of argument that he continues to be a total dick for the rest of his life. Whether I like his behavior or not, he's still living on his own terms, not being led about on a leash by bullies, crooks, or the world in general. That still works as a concept, to me.
If this were a non-children's film, and something much darker, then I'd probably buy that. But the problem is that this film is aimed for little kids. What's a five year old going to think about Ichiro's actions at the end? That bullying is okay as long as it's funny? That as long as you stand up for yourself, you can do whatever you want, including harm other people? I just don't think those are good things to teach little kids.
The question is whether one thinks the message of the movie is "stand up to bullies, but be a nice guy otherwise", or "stand up to those who would dictate your desires, and choose your own identity". If the moral is the latter, but Ichiro pranks the painter because the bullies wanted him to do so, then the message of the film is flawed. If he does it because, regardless of the reasons, he wants to do it to satisfy his own urges, then the message stands.
I find it hard to believe that Ichiro did that to satisfy his own urges. This kid as has seen other people get bullied and how it makes other people miserable. Not to mention he's been in that situation many times. He knows what feels like to be on the receiving end, and it sucks.
When I was his age, and I was being bullied, I also knew what it was like. And I thought that no one should ever be in those kind of scenarios. If someone has been bullied enough, then I'm sure that they would do whatever it takes to never become a bully, or to make sure they never bully someone. Yet here's Ichiro, someone who knows what it's like to be bullied, making someone's life miserable.