Well this is a thread asking for well written female characters, not necessarily nice ones.
A lot of the things you're complaining about in regards to her character are kind of the whole point. Yes she's cynical and judgemental. She's given up on pursuing things with passion because she's grown to see her own interests as small and insignificant and views others through that same lens. She judges them because SHE has long been enlightened to the fact those desires are silly, often unattainable, and not worth getting stressed and emotional over.
A huge part of her character growth is all about her getting more involved with people, being influenced and inspired by them, to the point that she can no longer stifle her own feelings and deny the fact that she actually really does care and wants to achieve things and shows emotional vulnerabilities she wouldn't have before. She ends the show very much caring more about both other people and herself.
I'll say though personally there are very few female characters I dislike because they are "bitchy", just because it's such a blanket insult for people to throw at female characters for just about any reason. I would not even consider Kumiko a bitch myself. Her "bitchiness" is not something she uses outwardly to hurt other people, it's the internal justifications she makes as a defense mechanism for herself and it's not glorified or treated as the correct worldview. So is it really such a bad thing? I don't think so.