Godzilla also has a certain level of durability. In the MHA world, quirks that can penetrate or bypass such extreme durability would be highly valued. On the other hand, Godzilla's presence could inspire the development of new support items or quirks that are specifically designed to deal with large and powerful threats like him.
Godzilla might see the pregnant Muto as a threat. After all, Mutos are his enemies. So he could start hunting it down to prevent more Mutos from being born.
Naruto would probably be both amazed and a bit scared at first. But being Naruto, he would quickly jump into action. He might try to use his rasengan against Godzilla, thinking it could do some damage. However, he would soon realize that Godzilla is much more powerful than he initially thought and would have to come up with a different strategy, like using his speed to avoid Godzilla's attacks and find its weak points.
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.
Shinji would probably be terrified at first. He's already hesitant and scared in his normal battles, and Godzilla is much larger and more intimidating than the Angels he usually faces. Asuka would likely be more aggressive, thinking she can take on Godzilla with her Eva. Rei, on the other hand, would be very calm and observant, trying to understand Godzilla's nature. Misato would be in a panic initially, but then quickly start strategizing on how to deal with this new threat. Gendo would see Godzilla as either a new opportunity or a major obstacle depending on his long - term plans.
Some of them might be amused. For instance, Hulk could find it funny if Godzilla was shown as being weak in the fanfiction. He might laugh and say 'That big lizard not so tough!'.
Some of them would be amused. Hawkeye might chuckle at the outlandish scenarios in the fanfic. Black Widow would be more reserved but still interested. She'd be quietly observing and thinking about the implications of the story. Captain America would try to find the moral of the story, if there is one, as he always does.
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.