It sounds like your older one just needs more guidance. I assume she is still living with you. What you can do for her is be very specific about what she needs to do, instead of throwing out general guidelines. She is probably less able to organize her life, but still capable of following a good routine that someone else establishes for her.
My younger sister and I had a comparable situation when we grew up. My sister was impeccably organized, and I was not. My mom would tell us to clean our rooms, and my sister's was pristine within a few minutes. Hours later, mine was merely reorganized, but not improved. It just didn't come naturally to me. I would spend a lot of time and effort on it, but getting my belongings (which were numerous) to fit in my room (which was small) in a manner that was visually pleasing to my parents was not a skill I possessed. Sometimes my younger sister felt sorry for me and did it for me. Ultimately, my parents just had to be specific about where they wanted things, instead of expecting me to "figure it out," then have them assess it. Once they organized my room according to their standards, I could easily maintain it. But every time my parents wanted me to put something new in my room (they were somewhat hoarders), they had to be specific about where it went. And that I could do.
It may be that your older daughter simply doesn't visualize body image, style and hygiene the way you do. Can you help her put together some outfits that look neat and classy, so she doesn't have to figure out what goes together? Can you establish a routine for chores that addresses her clothing and hygiene needs, so she doesn't have to plan ahead? For example, require showers, hair/teeth brushing and such in the morning before breakfast; require laundry duties two days a week after she gets home from work/school or before dinner. I think by age 20 it's unlikely that she'll initiate these life skills on her own, so you are probably not depriving her of her independence by prompting a routine that addresses issues she is neglecting. I think weight management is an entirely different battle and a very common problem. Short of strict adherence to a specific nutrition and exercise regime, most people do not learn to lose weight. For that matter, you should either dismiss your concerns about her weight or enroll her in such a program. Exercise classes can be really rewarding and have helped me get back in shape several times, but you have to want to do it to get anything out of it.
I know a family with five talented children and one daughter with Down's syndrome. For them, it was not about getting her to achieve the same things as the other five. They focused on what she could do, and worked with her very closely to help her become a certified dog trainer and groomer. She now works at a kennel and is very good at what she does. As far as I know, she does still live with her parents and probably will always rely on family to some extent. The point is that your daughter can be happy and well-adjusted without being everything or anything that her younger sister is. I don't think you should diminish your younger daughter's accomplishments to "even things out." God made them different. You don't need to pretend they are the same. Encourage both of your daughters to do what they are good at, instead of focusing on getting them to feel equal. Believe it or not, the discrepancy probably bothers you a lot more than it bothers your oldest. Few siblings aspire to be like one another.