I just prefer to take the point of view that, in order for Mal to have a character arc in the movie, they had to quietly ignore all the character development he had over the series. It's jarring, but what are you going to do? Though his short-temperedness is explained in the movie by suggesting they're going through a very rough patch, and having a lot of trouble finding enough work to keep the ship gassed up and the crew fed.
And don't talk to me about Better Days. It was fun, but it had no shortage of problems.
1: It's yet another adventure in between the movie and the series. There just wasn't that big of a time gap, and I'm sort of afraid that, after killing off two characters in the movie, Whedon's just going to keep jamming stories into that gap instead of moving on into the future. The other problem, of course, is that the explanation for Mal being such a giant asshole in the movie is that they're in a lean patch, so he looks like somewhere between a moron and a dick for dumping all that money, considering that a month later, he's going to be shooting innocent people because he can't afford to botch a job for moral reasons like he used to.
Actually, now that I think about it, that might actually help explain movie-Mal. He's really angry at himself, because his romantic gesture to preserve the family and life he's built on Serenity ended up actually dooming it.