Star Trek: Nemesis capped off two bad movies from the Star Trek: The Next Generation crew, with the first one being Star Trek: Insurrection. Nemesis was about a clone of Jean-Luc Picard, played by Tom Hardy and named Shinzon, who was dying and needed Picard to help save him. It was a bad idea and it was poorly executed, and even franchise legend Jonathan Frakes saw that.
In a passage from In The Fifty-Year Mission: The Next 25 Years written by Edward Gross and Mark A. Altman, Frakes recounts why he believed Nemesis failed, despite all the right elements being added to the project, with Frakes saying;
"With 'Nemesis' it was as if the Fates stepped in. We had John Logan, who's an A-list writer. We
had what was a great company back together again. Tom Hardy was a brilliant guest star. My take on it, though, was that there was not enough of the family. It was a little too much of Shinzon and not enough of Picard and Data. The first weekend people came to see Bones, Kirk, and Spock, or Picard and Data, you know what I mean? The story of 'Nemesis' was very much a story about the obsession of Shinzon, Tom Hardy's character. The front end of the movie with the wedding of Riker and Troi, and which was so charming, just got cut to bits. We originally had [returning actors] Whoopi [Goldberg] and Wil Wheaton and all of these Easter eggs in there, and Brent [Spiner] sang. It was a big deal that got cut up to nothing."
Now, he's not wrong about the issues. He's very correct about the reasons the film flopped. It's worth noting, however, that it wasn't any of those elements that caused fans to turn away from Nemesis. It wasn't how little we saw the "family", the returning characters that got cut, or the amount of screen time Shinzon got.
It was the entire gimmick. The whole story of Shinzon being a Picard clone that the Romulans were going to use to attack the Federation seemed so far-fetched and almost self-aggrandizing. How big is the ego of Patrick Stewart/Jean-Luc Picard, that he would be the key to the downfall of the Federation?
It felt so silly, and was presented even worse. Some concepts just can't work as a film and the "evil clone wants to kill you and replace you" bit was overplayed long before Nemesis. Sadly, the film was doomed to fail simply because the story sucked that hard.