As a dad with three daughters, I do sypathise deeply, and understand completely.
It's called adolesence!
At this age, girls dip out of the human race, which makes them impossible to live with. The good news is, they do come back, but not just yet.
So, you see your daughter not looking after herself, and determined to slob about in dirty clothes - and you don;t understand how or why this can be, given the way you have brought her up.
OK, it's simply rebellion. Your daughter is going through massive physical and emotional changes, and this is her way of coping. Alienation is the top of her list, she feels she doesn;t fit in anywhere - home, school, friends, so she reacts by dipping out of all the aspects of life that show her belonging.
As her mum, you get this approach for longer, and with more intensity, than anyone else. It looks as though she is simply doing this to upset you, which in a way she is, but it's not meant in as nasty a way as it appears.
As you have found out, trying to butt heads simply gets you into a war of attrition, which underlines her feelings that everyone is against her, and makes you and her miserable.
What you need is a different way of appraching her. She is going to be a young woman one day, and a child the next, you have to roll with it. On the 'rebel' days, let as much as you can go without making an issue, and make a seriously big deal of being positive if there is any sign of mature behaviour.
Try and spoend one-to-one time with her - either go out together if she will, not shopping, she will be seen by her peer group - try the pictures. Maybe watch a DVD or TV programme togerher. Ask her at regular intervals if she is OK, and has anything she needs to talk about. Ignore completely the hostile response you may get - if you catch her on the right day, she may just open up and tell you what is going on with her.