That's the biggest problem with Zack Snyder's films. They're not fun. Now that doesn't mean you can't be serious and dark, we all know what happens when a movie goes too far in the other direction...
Take a look at the first Avengers film. Throughout a massive battle on which the survival of Earth hangs in the balance, the heroes are still able to smartly quip at each other or even crack a joke. Even Captain America: The Winter Soldier which is perhaps the most serious of the MCU films still finds time to for fun and bits of humor. These bits of humor don't take away from the film, in fact they often help it out. If you have constant dismal darkness (cough cough Man of Steel cough cough) The audience will get so bogged down by the sullen teenager that is your movie that they won't be able to enjoy it.
There are however instances where trying to be both dark and fun backfire.