They absolutely should.
First things first, outside of the TDKT none of their superhero movies in the past have been "dark and gritty". Look at the early Superman and Batman films, look at Steel, Jonah Hex, Green Lantern, Cat Woman, Superman Returns, etc. I despise the notion that DC makes "mature, realistic" movies because it is simply not true; Nolan makes those kind of movies, and he happened to direct the most successful DC property in the past twenty years because it fit that particular character.
Now, that brings us to Man of Steel. That film was clearly trying to look like it was cut from the same cloth as Nolan's trilogy. They obviously wanted to take the same approach that had been successful in the past and apply it to their most recent effort, and in my opinion, the film suffered for it.
Superman is not a dour, humorless character. It was a giant mistake right out of the gate to try and make him fit in the same mold as Batman because as Cyclyopswasright stated, the character should decide the tone of the movies. I'm not saying that a dark Superman movie can't work at all, but I am saying that tone is not particularly suited to that character and his mythos and there are many more appropriate routes to take.
DC is apparently making the default aesthetic of the movie "dark" or whatever (I honestly hate that word, it's so overused) which, in my opinion, just shows how myopic and misguided they are in their approach. "It worked once, so let's try it again and again!"