IIRC, Nemesis was beaten on its opening weekend by Maid in Manhattan. The only movie to blame for Nemesis’s poor box office is Nemesis.
I'm not sure that follows. Opening-weekend box office is shaped more by advance buzz than by reaction to the actual movie, for reasons that should be self-evident. If a movie is bad and gets good buzz, then it will open strong but then drop off rapidly after the opening weekend, as happened with, say, Batman v Superman. If even the opening-weekend box office was weak, that suggests it was more due to a weak marketing campaign, or a lukewarm response to the previous movie.
I do still think there being basically 3 blockbuster films out within a few weeks definitely had an effect on Nemesis. Partly because the people that would go to see a Star Trek film are probably at least some of the same people that would see a Harry Potter film, a Lord of the Rings film and even a James Bond film. They do tend to pull from the same pool of fans. And while I personally liked Nemesis, I'm also self aware enough to know that it's not going to pull blockbuster numbers. And people are only going to see so many movies in a month. I think oversaturation of franchise fantasy/action films had a downward effect on Nemesis' returns.
Whereas "Maid in Manhattan" I would think pulled from a different pool of fans. I would argue many of the people that went to see "Maid in Manhattan" are not really the same people that would go to see any Star Trek film, or even necessarily a Harry Potter film, a Lord of the Rings film or a James Bond film. So I think it's not the best movie to compare to.
And Christopher has a point too. I knew Nemesis was coming out because I'm a Trekkie. But much like Star Trek Beyond, I do remember Paramount put a minimal effort on promoting Nemesis. Now Nemesis-haters would say that's putting good money after bad---but if you do a poor job promoting a movie, you can't expect it to do well.