There is no reason a dark superhero movie can't be good. The problem is that some productions have assumed that only dark superhero movies are any good and have applied that darkness thoughtlessly to the wrong characters, or as a substitute for having something worthwhile to say. Man of Steel made the mistake of beginning Superman's story with darkness; he's a character who should initially come from a place of light and hope, so that if a dark story befalls him later, the contrast is more meaningful. Then there's the misbegotten David Kelley Wonder Woman pilot, which totally misunderstood the character, assuming that Wonder Woman was just a generic violent, angry vigilante rather than attempting to understand what's distinctive and important about her.
The thing is, there was a time when telling superhero stories in a darker, grittier way was an innovative departure from cliche, and so it was often impressive as a result. But people took the wrong lesson from it and assumed it was the darkness rather than the innovation that mattered, and so now, 3 or 4 decades later, dark-and-gritty has become the lazy, cookie-cutter approach, the first resort of the unimaginative. Of course it's still possible to make a darker movie work -- Captain America: The Winter Soldier is probably the darkest MCU film and probably the best one as well, but that's because the franchise had adequately established Cap as a figure of hope and positivity and managed to put him into a darker situation without losing that. The problem is when you start from the assumption that darkness is the only option, or that it will automatically make your story work.
