He might be amazed. Naruto is used to his own world and seeing a fan - made interpretation like an AMV fanfiction would be like seeing a new side of his story.
It's highly unlikely. Naruto's world is preoccupied with real - life ninja stuff. Fanfiction and AMVs are products of our modern - day, media - savvy world. There are no computers or the Internet in Naruto's world for him to access such fan - made content.
Well, Naruto might be really confused at first. He's from his own world and an AMV (Anime Music Video) fanfiction is a very modern and fan - created concept. He could be intrigued by the different interpretations of his story or his friends' stories in the fanfiction.
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.
He could also get angry. After all, being misjudged like that is unfair. His anger could lead him to either lash out verbally at Sakura or he could leave the village in a fit of rage, feeling that he is not accepted even by his friends. This could then lead to a series of events where Naruto has to find his own way and Sakura realizes her mistake later on.
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.
Kakashi would be the most perceptive. He'd see how Naruto singing 'Monster' is not just a random act but a way for Naruto to connect with his own fears and insecurities. His friends from the Hidden Leaf Village would overall be proud of Naruto for trying something new and for using the song to share a part of himself that they may not have fully understood before.
Sasuke would likely be deeply affected. He might initially be angry that he couldn't protect Naruto. As Naruto becomes a monster, Sasuke would be conflicted between his desire for power and his loyalty to his friend. He might try to find a way to bring Naruto back to normal so he can have his rival back. Sakura would be heartbroken. She would probably cry a lot and then try to find a medical solution to reverse the transformation. Kakashi would see it as his failure to protect Naruto and would do everything he could to stop the monster Naruto from causing harm while also looking for a way to save him.