Good writers know that their writing must be true, and thus it must include humour because humour is a part of life. Also that the humour must be in-character, not any old joke that will raise a laugh – then it’s not cinema, it’s panto. Thor’s comments in the first movie are funny because they are how an arrogant Asgardian god might see our world. But six years later: ‘He’s a friend from work,’ is the director* sneering at you for taking superhero movies seriously.
That’s the lazy way to get a laugh, which is just to have characters in a fantasy setting use slangy modern idioms. But the writers who began the trend did it with serious intent; they still wanted you to believe in and care about their story. They were looking for ways to make the audience relate to the characters, and clearly lots of ‘Prithee, varlet’ dialogue wasn’t going to do it. There is plenty of humour (I hope you will agree) in Mirabilis, but Leo and I try never to put a line in a character’s mouth if it isn’t true to the moment and spoken in their voice.
Undercutting tension with humour can be very effective if it’s true to character. Look at Steed and Mrs Peel, most especially in the scene at the end of “The House That Jack Built” when the defence mechanism of their insouciance almost breaks down. But for the writer who doesn’t care, it’s a short step from there to having every character reach for the glib line that will get a laugh.
It is unjust to call this MCU writing. The entire Captain America trilogy managed to include humour in a way that rang true. The Russo brothers’ Avengers movies likewise. And in any case, Marvel didn’t invent the trend. Look at the Universal monster series. They start off selling us the story straight with Dracula, Frankenstein, The Mummy. Four years on, The Bride of Frankenstein is most definitely Whedonesque – or perhaps we should say that Joss Whedon’s writing is Hurlbutian. It took Universal a bit longer to get into their non-stop gag phase but Abbott & Costello Meet Frankenstein could easily match Thor: Love and Thunder or Willow for hey-it’s-all-a-joke silliness.
* Yes, we all know that particular line was suggested by a kid who was visiting the set. But it's the director's choice whether to include it, and it fit in with the tone he decided on for the whole movie.