I don't think Miyuki and Sakura are alike at all. They're totally different.
The reason Miyuki went to see the soccer playboy because Eiji didn't kiss her, just holding hands, and she felt bored.
Miyuki took Eiji for granted. She thinks he'll always be there for her. So, she thought she'd enjoy her youth (sex pleasure) with the soccer playboy till he graduates then she'll return to being Eiji's good girlfriend. Selfish, isn't it?
However, her plan collapse when the situation came to light. Then she made the wrong choice, sticking with the soccer playboy and framing Eiji to hide her infidelity from their schoolmates. She never knew Eiji would commit suicide until Aijung told her.
Miyuki isn't stupid but selfish and deceitful. After all, a stupid person couldn't be one of top students.
Sakura, on the other hand, is so stupid that readers simply can't stand her.