I think Hinako does possess beauty according to the ideal beauty standard. Perhaps her features are a mix of mixed ethnicity, and when Shosuke first saw her, he probably hadn't seen a woman with a face that deviated from the ideal Japanese beauty, which is why he fell in love. (You might wonder why I wrote that she's mixed race. It's because, based on her observations of her younger sister, Zeniko, who has blonde hair, it's speculated that her family has European ancestry, likely meaning the father of her two daughters is European.)
Think that her unique beauty, typical of Japanese girls, might have led her to experience similar things to Shoko – excessive attention from classmates, even stalkers. She seemed to disagree with and dislike this, so she strongly reprimanded them, telling them to know their limits and respect her. Luckily for her, no one bothered or pampered her excessively; instead, they respected her or feared her from then on. But she still faced bullying because of her beauty? Her friendship with Shoko, combined with her somewhat naive or forgetful nature (though she's actually very intelligent), might have contributed to her depression, panic attacks, and loneliness.