That certainly is not it. I found Dredd to be mediocre, and ASM to be terrible, but beyond that, you're forgetting 300, 30 Days of Night 1 & 2, Blade Trinity, The Spirit, both Punisher films, Daredevil, Surrogates, and Priest. Tiptoeing outside of the genre, we have exemplary films like Immortals, Clash of the Titans, Underworld, and The Wolfman. Surely I've missed several, but you get the idea. Let's try to be a bit more impartial, shall we? In any case, my point is that tone is not and should not be the most pivotal barometer for judging a film, but time and time again I see folks dismissing certain movies for no other reason than the lack of a dark tone. That's nonsense. In the case of many of the films I mentioned, it's quite clear that the producers sacrificed substance for style in their approach. A movie should never get a free pass for being excessively serious, filled with bleak overtones, and angst; those aspects, like anything else, need to be done well first.
I disagree. For one thing, all of those films are definitely not masterpieces; at least half of them being the furthest thing from such a designation. Anyway, I believe it is becoming a fad because many fans seem to default to "dark & gritty" as the primary criteria for a film before anything else. Check interviews from filmmakers and industry people, and they often times echo the same sentiment - "we want to approach film XYZ as a more serious, dark and grounded affair." In my opinion, this is approaching things in reverse. Tone should match an already established story, setting, and characters, not the other way around.
Again, I don't believe this to be unequivocally true, or even remotely so. Your last sentence is the main issue I have with this approach. Your logic implies that since a dark film takes its subject matter more seriously(which isn't necessarily accurate, but I'll concede the point), this results in a better film. That simply is not true. Seriousness for the sake of it is not quality, it is simply an approach. Said seriousness needs to be done well, and be anchored by an otherwise solid film. I'm reminded of other filmmaking approaches that audiences and critics both tend to give free passes to. True story, 'feel-good', and uplifting films are among the worst offenders(We Are Marshall, I Dreamed of Africa, Seven Pounds). Realism is another. I always roll my eyes when I see realism listed a selling point for a movie, because realism, in and of itself, does not and cannot automatically impact quality. My point is that tertiary approaches and characteristics should not be held in higher regard than the actual content and substance of a movie.
In any case, don't assume that I'm pulling for light over dark, or funny over serious, because that's not the point I'm trying to make. I simply would prefer that films and fiction in general be approached with storytelling and characters in mind before style, mood, tone, realism, or whatever the flavor of the day happens to be.