I think Carolyn suffered from having too much character and not enough time to grow into it. She was a typical angsty teen with a secret, and it wasn't until the climax that the secret was revealed--and it was so unnecessary to the plot that it was disappointing. Part of the downfall of that movie was that there were too many layers that could've been just left out and explored in subsequent films. And, of course, the comedy included where it really shouldn't have been.
I like the movie, and I watch it here and there, but it always gets me how much it should've really shined compared to what it was. I do think its interesting that you point out that Carolyn in the movie hues closer to Elizabeth in the series.