Well, I've read most of the Amazing series (#1 - 350 or so) and around the 100s, he really is pretty serious. I agree that during the spider-man fights, he should have quipped the funny more with minor battles (the fights with Doc Ock that were the majority of the action stuff was pretty dire circumstances and I don't think quipping during those situations would have been in-character... spidey usually cracks funny when he's in control more), but during the peter parker scenes, it seemed to fit well with the early "parker in college" times from the early Amazing series.

I suppose it might've seemed too serious because they were using a lot of the reasons Parker's life was in shambles in a 2 hour movie rather than over the course of a 100 comics.