The adjective slouchy usually describes clothing that fits in a loose, unstructured way, like your favorite slouchy sweatshirt or the slouchy socks you wear to bed.
You can describe people with stooped postures as slouchy: "The substitute teacher nervously entered the classroom full of slouchy teenagers." But it's far more common to talk about slouchy boots or a slouchy, hand-knit sweater. Slouchy comes from the noun slouch, which has a Scandinavian root and meant "lazy man" before it came to mean "stooping of the head and shoulders" as well.