Guardians of the Galaxy is a bad example. I don't want that to be the gold standard of comicbook films for either company.

The film is supposed to be silly, it was never about the end of anyone's world, it was always about the main 5. They could have put a pineapple there as a villain and the film would have been exactly the same. It's a purposefully ridiculous film.

But it felt like every 5 minutes there's a joke, even when it wouldn't fit, even when it doesn't match the person . So certain scenes never play out to their length as a result. I'd argue never took its premise seriously enough to have a villain that's a threat. I never really felt like their world was going to end and more importantly I never felt like it obeyed the rules of its own film. It sacrificed a lot to make its gag and a lot of it doesn't quite land.

"Fun" does not dictate the quality of the film. "Fun" doesn't mean automatically mean good and its dangerous to conflate the two. Whether you enjoy it or not has nothing to do with fun.

Maturity means its appropriate, like childish means the opposite. (And all grit means is its realistic or relatable, unfortunately now its a shorthand for gloomy these days but whatever. So everytime you see the word "gritty" think "human" instead)

Superman cracking a smile when he's flying. Appropriate and joyful. Superman being sassy during an interrogation. Appropriate and it lightens the scenario. Superman cracking a joke while the buildings crumble. Inappropriate and jarring.

Rocket Raccoon mocking Drax when he's whining about his parents. Appropriate, funny and works since Drax was being a little selfish. Drax calling Gamora "this green whore" during a speech about friendship. Inappropriate.

I agree with your point with the DCEU needing more levity after MOS, but I disagree that it needs more jokes. There are lots of ways of lightening moments without quipping. In fact, there are lots of different types of humour. The MCU has relied pretty heavily on the same kind of verbally delivered joke, for everyone. So when Thor, Captain America, Tony Stark, Hawkeye and goddamn Phil Coulson are telling the same kind of joke, it doesn't differentiate them. In fact it paints them all with same kind of brush and without the visuals you wouldn't be able to tell where the joke came from.

Part of what made me so excited about Ant-Man and best parts were the different kinds of humour. The fight on the train set? The fight in the suit case? Pure physical comedy and it fit the world, it fit the characters and it builds the story.

Humour especially when verbally delivered, like all dialogue has to give me insight into who the character is. It has to push the narrative forward.