Bakugou might react with his usual anger. He could lash out at the bullies verbally or even physically, which might make the situation worse. For example, if someone makes a snide comment, he could respond with a loud and aggressive retort.
Well, there's the theme of sacrifice. In some fanfictions, Bakugou's mom has to make sacrifices for her son's future as a hero. This could be sacrificing her own time, or in some cases, her own safety. Another common theme is her influence on Bakugou's values. She instills in him the values that are important for a hero, like justice and perseverance, and fanfictions often explore how she does that in different ways.
Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
One way to cope is to write down your feelings. You can keep a journal and write all the things that make you hate her and the sad parts. This can help you get it out of your system. Then you can think more clearly about how to deal with it.
If it makes you uncomfortable, you should talk to her. Just say something like 'Mom, these are my personal stories and I don't feel okay with you reading them.'
First, it's important to respect her privacy. You could try to act as if you haven't seen it. Everyone has their private side, and this is a normal part of an adult's life.
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.
Well, sometimes people feel this way because of misunderstandings. Maybe your mom has high expectations and it makes you feel pressured. For example, she always wants you to get top grades but doesn't understand how stressed it makes you. But often, these feelings can be resolved through communication. Try to sit down and talk to her about how you feel.