There's motives which could be interesting as HaventGotALife said, but I think unfortunately they are given a shallow presentation.
The movie might have gone in a more interesting direction if Shinzon actually did stop acting like a two-dimensional villain and attempted to ally with Picard. The most interesting parts of the movie are when it seems like this might be possible.
As it is, it feels very much like he's shoehorned into acting like a cookiecutter villain. All of that might not have been so bad if he actually acted at all like Picard. He neither looks, nor acts like Picard, his only relation is that the movie tells us he's a clone. It really lets down the "mirror" aspect of the story.
So yeah I very much agree Nemesis would have been better if Shinzon was better. It's not as godawful as sometime said but I'd say it's tied with Generations for worst TNG movie.