I think First Contact's theme (which is great) often gets conflated with its entire score. Goldsmith wasn't exactly turning in masterworks in the last decade or so of his career, and outside of that majestic theme, First Contact's score is ... well, pretty close to the sonic wallpaper that so many people complain about.
In honor of the 20th anniversary I listened to the soundtrack over the weekend, and I agree with you that the First Contact theme, while lovely, was overused throughout the score. I would say out of the three TNG movies Goldsmith scored, Insurrection was arguably his best work.
Correct me if I'm wrong, but I seem to remember reading that Goldsmith was hired fairly late in the game and didn't have a lot of time to work on the score (he was also working on another movie at the time) which is the reason his son Joel wrote some of the tracks for the movie.