"With a few well-timed puns and a giggle or three, I WILL save Metropolis!"
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. You might hate a Superman that is a 50/50 split on frowns and punches, but "fixing" him by dribbling random pleasantness from his super-orifices is like finishing an algebra test by including a five-paragraph analysis of the works of Charles Dickens.
"Why don't they like me yet? I made the jokes. I did the pratfalls. I did everything they asked."
Also, we've already ragged on DC for copying Marvel's Expanded Cinematic Universe idea. If DC suddenly decides to start creating the kind of movies that Marvel is making in terms of how funny they are, we're just going to give DC hell for that too. I don't want every film that they come out with to have a tone like Batman v. Superman, which can best be described as "slow motion tears fall during slow motion rain drops." But I also don't want a series of movies that decapitate their own worth because of a misguided WHAT ABOUT THE FUNNY? campaign.