I mean, it's not the best manga in the world, but if anything, I blame it on how quickly it flows together. As a shoujo, it's typical-- "ugly" girl is not so ugly and transforms into someone more adorable. Of course, if she's grown up her whole life thinking she was ugly, her social skills would be terrible and she wouldn't understand basic qualities of friendship /relationships.
*** Consider the rest of this a general, non-specific spoiler***
The "dramatic transformation" isn't all that ridiculous considering her family. Her brother and supposedly parents are "gorgeous." It makes sense that someone related would have the same kind of beauty undertones. If she was actually ugly it'd be a bit unrealistic in truth. While the whole family doesn't have to be identical, it makes sense that she's pretty too.
Along with that, anyone who thinks her self-esteem is too low, you clearly don't understand the situation. If you're raised your entire life thinking you are ugly and therefore worthless, your self esteem would be in the toilet too. Kids today kill themselves for that kind of torment. Now if she had some good friends or something that kept her afloat, yet she still acted like that, I'd be mad. But she has nobody. No one talks to her, she's been "poison mushroom" for as long as she can remember, and she's been told her whole life would be like that. Even at home, her brother legitimately treats her like a slave because she's the black sheep of the family. Even if fixing herself up would make her pretty, as long as she's told she's ugly she'd always think that way.
When Kumada tells her that day to "work harder" it wasn't a message that says "look pretty and everything becomes easy." The point was that he saw her-- not a mushroom, not a shadow, her. He looked at her and talked to her, and that had never happened before in her life. I think Fuwari said "a girl wants to be pretty for one reason" and in shoujo world, it's because they met someone they wanted to be pretty for. It's the same thing except without the love aspect. She was told over and over again that there was nothing she could do about being ugly, (only exception was Fuwari who had a ton of social issues of "her" own) and here was a guy she had barely met telling her she wasn't ugly because she was her, but that she was making herself ugly. Someone out there believed in her and she didn't want to disappoint them.
By telling her to change, he jump started her to pick herself up from all the self-loathing, The old her would never have sought revenge, but she felt empowered. So what if it took a new hair cut and some make up to do it? What teenage girl didn't think at one point a little mascara could make her dreams come true? And of course, she still refers to herself negatively, what you'd expect from someone recovering from crappy self-esteem. And we all get pissy at the "don't leave friends for a boy" but think about friggin real life! Don't tell me that you weren't ridiculously into the first or second person you dated. Most of the time, you don't think you're neglecting a friend, but you do. The friends feel it, not you. It's a two way street, the girlfriend sacrifices bf time and the friend sacrifices bff time. It's an even pull, so why get mad because she ditched once or twice? I can think of tons of times when one of my friends started dating someone new and ended up blowing me off. I understood because we were friends. This chick is clueless with friends and dating so I'd be more put off if she actually knew what to do!
The only thing irritating about this manga is her character development. She needs to realize some stuff a bit quicker, but there are tons of shoujo like that so what makes this one significantly worse? She's cute and learning along side a set of witty 'friends keeping it all interesting to a degree. So yeah, it's a typical shoujo with a silly and awkward main character. Whoopdy doo. Not a horrible evil messaged story that ruins teenage girls.