I don’t think the tone of the DC movies so far has been what’s doomed them. Nonsensical plot points, scenes that start something and proceed to go nowhere, and pacing that allows them to wallow in their own shit for an hour before suddenly ramping the narrative through a building – that’s what’s turned these things from “movies” into “collections of half-formed ideas barely held together by themes that the characters constantly shout at each other.” And when that complaint was put through the I Need To Get This Down To 140 Characters Or Less Translator, it apparently came out as “Needs more jokes!”
The Marvel movies can be pretty funny. Ant Man, Guardians Of The Galaxy, and every line from Peter Parker in Captain America: Civil War are all humorous without feeling like someone went through the scripts with a red-tipped pen to make sure that these costume parades got some ha ha’s. And that feeling is super important, because it at least gives off the illusion that once we enter the theater, we’re being put into the hands of competent filmmakers who know how to manage a mood, and not the wallets of executive overlords screaming “GET ROBERT DOWNEY JR. FOR A CAMEO, YOU FUCK!” at every opportunity.
But adding lightheartedness to a movie is not a tonal Band-Aid. We already saw that applied to Suicide Squad, and that thing ended up seeming like a tug of war between two totally different Suicide Squads. Batman v. Superman would not have benefited from some sprinkling of whimsy. To create a movie that can handle a mess of jokes, you have to rebuild it as such from the ground up.