Sometimes women don’t care about how they look because they don’t feel good about themselves. If this is a problem, then show her more appreciation and give compliments as often as possible. In short, treat her well. Don’t know if this is the issue, but it’s a reason for some people who don’t take efforts with how they dress.
In addition, your wanting for her to dress better should be because you care about her too. If you just show that you want her to look good for your image or only for your enjoyment, then forget it.
(Just suggesting possibilities, not saying it definitely applies.)