Please note that this is a mild complaint and nit-picking. I think it was a great movie based on a great show. I knew going in, even the first time, that it was a movie and not a TV show. What you say about tight pacing is true, and I think some Firefly resent the fact that the movie doesn't have the more leisurely pacing of the TV show with time for banter and character building. But I think it's a great movie, and the non-Firefly watching non-SF fan that I went to see the preview with like the movie just fine enough to go see it again last night.
I was trying to be as open minded as possible. I really don't blame Jewell. Especially for the tacky line. I think the reality is that her character is simply one that needs a certain amount of screen time to be rounded out. What we have of her here is her edgier side. The one who has her dander up about a couple of different things - which is a perfectly legitimate emotion for that character and well played. but the Kaylee of the series was also more cuddly and affectionate ("I Love my Captain!") and there's just no time to show that.
As a firefly fan, I can appreciate that limitation and not blame Jewell, but looking through the eyes of a non-firefly fan, I could allow that someone would argue that she gave a less nuanced performance than some of the others. In reality, it's the dictates of the script but I wanted to give the benifit of the doubt to those looking for something less than perfect.