I think the problem is that, for many people, the title and trailers maybe made the movie seem a little TOO dark and gritty for their tastes.
With the themes of war and terrorism, it just didn't look nearly as light and fun as the first one.
Of course my theater was full and everybody burst into applause afterwards, so I think there's still a good chance the word of mouth will improve things. And I have NO doubt they'll make a 3rd movie-- it'll just be a lot less dark.
Then WOM will eventually filter through to the masses that it is not quite as grim as they believe and gradually they will turn up and watch the movie. We have to hope it gets good 'legs' to make it to $200m and well beyond.