the second thing I noticed I liked about anime, outside of the direct stuff you witness and in terms of the story, was that characters didn't just arrive on the scene with knowledge they shouldn't have. Even in Dragonball, characters would have to be fill in, often on screen.
When people say Miyuki is stupid, I can only imagine they seem to think that she should know these things only the viewers/readers are aware of. Like she should know Choko keeps gunning for her or talks behind her back. As far as she knows, Choko only did one thing, and that was apparently not tell her she was seeing kiiechiro/her senpai guy. And She actually apologized for that.
Is she stupid because she doesn't hate her brother? Her brother is a total screw up but whether she hates him has nothing to do with intelligence. Nor would it let her out of her debt. Not sure how the law works but if he was the guardian of the family, his debt would go to the family as a whole. But even if it didn't, she's dealing with gangsters who want someone to pay. It's not like her position is affected by being angry at her brother, as justified as that would be.
Same thing when she stayed at that lady's house, she needed a home, and working for food and board is just what the situation was, a bad attitude wouldn't have changed it.
The only valid point one could make is that she doesn't like talking to the press. But it's not like that couldn't blow up in her face or as if her association with yakuza and Thunder/her banned coach, wouldn't help feed negativity. Or how easily wouldn't any of the people that dislike her be able to call her a liar ect.
edit: There was that horrible contract she signed too. Maybe that was naivete but how many 17 year olds wouldn't be tricked by that, and no one even tells her, and it isn't like she could have afforded a lawyer to read it. Also this was in 1993, when this sort of thing was still coming to light.
She is stupid because she literally cannot read the intentions of a single person around her or make sound judgments based on the horrible things that continuously happen to her. Not being able to learn a lesson, even though you encounter it again and again and again, would qualify you as stupid. or at least lacking any wisdom to speak of.
Take her relationship with Keiichiro. She walks in on him boning one girl, runs into him with another girl in hotels on more than one occasion, he abandons here at the most important moments, and can't keep any promises. And yet?