Poshmark closets are sorted by default using the "Just Shared" sort order. As a result, your listings will be sorted in reverse share order (oldest shares are at the bottom of your closet).
If you want to organize your listings by category, you will need to share the listings by groups (e.g., sharing all sweaters consecutively will organize all sweaters in reverse order). Unfortunately at the moment, we don't have a way to pin listings - although we plan to address this on our Product Roadmap eventually.
Bear in mind that most listing traffic comes in through non-closet channels (e.g., Brand or Search Page), so while your closet should certainly be organized the way you'd like it to be presented, the majority of buyers won't see your closet page when they visit your listings!