I think there are two reasons the designs can come across generic in the prequels.

The first being because there is so much of it thrown around on screen. Each design might look great on paper, but when there is so much happening in each frame it just becomes an orgy of visuals (which are often very similar) competing for your attention.

I think the second reason is the fact that these were the first films that really relied this heavily on CGI, and as great as it was for its time, it has a tendency to come across as very flat- especially in those ground based scenes with CGI backgrounds and landscapes, where things tend to blend together and nothing pops (Coruscant and Geonosis scenes come to mind). The color grading also plays a part in this. Although the space scenes tend to fare a little bit better in regards to this.

That being said, I do think there are at least a handful of designs which could be described as generic at best.

Some examples (in my opinion)…

Generic- Most of the Droid Army
Bad- RotS Jedi Starfighter
Destinctly not Star Warsy- Grevious