I really like the fan fiction 'All Might: The Early Days'. It delves into All Might's backstory, exploring how he got his powers and the challenges he faced before becoming the symbol of peace. It gives a deeper understanding of his character.
The mom could be shocked at first but then gently explain to the son that it's a private and natural but not something to be begged for. For instance, she might say that farts are a normal body function but not something we should play around with in this way. She could also use this as an opportunity to teach the son about good manners.
Bakugo might initially be in denial. He's always had a complex relationship with Izuku, and seeing him in such a state could shatter his world view. He might become more withdrawn or, on the contrary, be spurred into a violent rage against whoever was responsible.
Archive of Our Own (AO3) is also a great place. It has a large collection of fan fiction, including many about All Might. The tagging system makes it easy to find exactly what you're looking for in terms of genre, length, and more.
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.
On the other hand, the son might try to adapt in a way to please his mom at first. But as time goes on, he will likely start to feel a great internal conflict. He might start to secretly explore his true identity while still pretending to be a girl for his mom. This could lead to a lot of secrecy and emotional turmoil within him as he tries to balance his true self and his mom's expectations.
The boy might be confused at first. He may not understand why he is being dressed differently from other boys he sees. For example, if he goes to school in a dress, his classmates might stare or make comments, which could make him feel self - conscious.
The son might initially be very confused. He's a boy and being forced into a 'daughter' role is something completely alien to him. He may resist, not understanding why his mom wants him to do this.
Mineta could react with a mixture of embarrassment and defiance. He's a proud character in his own way. So when All Might yells at him, he might try to put on a brave face and argue back a little. But deep down, he knows he's in the wrong. Eventually, he'll break down and start crying, realizing that he has disappointed not only All Might but also his classmates who are all striving to be heroes.
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.