I'm not entirely sure as I haven't read it specifically. But generally, it might be about spies who go undercover or are in some form of disguise to carry out their missions, perhaps with lots of action, mystery and intrigue.
It's about a spy who gets turned into a pigeon and has to carry out his missions in that form. It's full of action and humor as he tries to save the world while being a very unexpected kind of hero.
It's likely about the story in the 'Spies in Disguise' world but in a junior novel format. Maybe it focuses on the adventures of the spies and their disguises, and the challenges they face while on their missions.
I'm not entirely sure, but it's likely to feature the main spy from the 'Spies in Disguise' story, along with his sidekicks or colleagues who help him in his missions.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
Some MHA characters might be flattered. For example, Midoriya would probably be really excited and take it as a sign of how much he's inspired people. He might even look for positive lessons in the fanfiction to improve himself.
If the MHA characters were to react to a Mastermind fanfiction, some would be more accepting than others. Characters like Ochaco might find it really exciting. She's a fan of the heroes herself, so seeing different takes in fanfiction would be like getting a new perspective on the MHA world. However, characters like Shoto, who are more reserved, might be a bit hesitant. He might need some time to process what he's reading. But overall, it would be a very interesting scenario as it would show how the characters view different interpretations of their world.
Some characters would be shocked. For example, if the fanfiction had a really dark and unexpected Mastermind plot, like a hero turning out to be the mastermind of all the villains' plans, characters like Tsuyu would be very taken aback. She'd probably question the motives behind such a plot. Then there are characters like Denki. He might be more into it if it had some cool action scenes in the fanfiction, regardless of how out - of - the - blue the Mastermind storyline was. And Nejire would probably be super curious about the emotional depth of the characters in the fanfiction.