Sincerity may only be able to get you a cup of coffee if you pony up about $2.50, but it can get you a lot further in the stories you write if you apply it to your characters. What I mean by this is having your characters believe in the world they live in, and thus are sincere in their interactions with it. This helps suspend disbelief and hooks your reader.
This may sound obvious, but consider a great many MCU movies you might name. How many times have you seen characters almost turn to wink at the camera, trying to bring you in on a joke being applied to the events on screen? It’s almost as if they know they’re in a story, that none of this is real, and that everything you’re witnessing is pretty silly if you really consider it.
It’s essentially characters treating their world, and the stories taking place within it, with a kind of irony that removes tension and lessens emotional impact. It’s all a game, and they’re bringing you in behind the curtain to laugh at what’s happening. In moderation this might be all right, but when overused it can break suspension of disbelief. If the characters don’t really care about what’s happening, then why should you?
Which isn’t to say characters can’t comment on what’s happening around them. Terry Pratchett, for example, played around with genre tropes all the time and his characters frequently poked fun at them. But his characters also still believed in the world they lived in, that what was happening around them was important. The characters were never mugging for the camera. In Pratchett’s work the narrative itself may address the reader, but you’ll never get Tony Stark constantly turning to Captain America to talk about how insane whatever they’re dealing with is in quippy dialogue.
If your characters believe in the world they live in, even if they gripe about it, then it will be easier for your readers to believe in it as well. It will feel real because the characters treat events as real rather than as a joke, and you’ll ultimately create something more likely to be remembered than dismissed with a laugh.