I can actually forgive the humor because, the way they set Britt up, it actually made sense for him to be a partying slacker. He essentially was pushed by his father so hard that he did the exact opposite to avoid becoming him.
Therefore, being silly throughout the movie because he doesn't realize how serious things are, in contrast to Kato who does, works ok. And it does have it's serious moments. It does go unnecessarily over the top at the end. But the moment when it all clicks for Britt and he goes badass is pure awesome. I just wish he hadn't of fallen at the end, instead helping Kato in some way that leads to the same result.
But this means that, in the sequel, Britt should have learned his lesson and be much more serious. At least in the Green Hornet persona, since he's supposed to be a crime boss. I'd still expect some jokes and humor, more so when he's Britt, if for no other reason than to try and maintain his cover as careless playboy.