In general I agree, but I really think it's a bit more specific to the character in question. Batman is unusually well suited to grimdark, though it's not a requirement. Indeed you'd have to dial it back a notch if he was working with Superman. Supes, of course, is (or ought to be) almost exactly the opposite, a superior godlike being constrained only by American values and occasional Kryptonite. He has to be a goody two shoes or his character doesn't work. This is what the MCU realizes--the story is driven by the inevitable conflicts between the characters, but those conflicts generally don't overcome the basic optimism and goodness that they all share. The DCU's fundamental problem is that they're getting the characters' personalities wrong, and the personalities are what's supposed to drive the plot, so the plot is all wrong too.