I wouldn't call her thin, that's what my sporthorse looks like. My boy is a bit ribby when he stretches out(still working on getting his weight up), but he's part TB so I don't expect much different.
If you think she's thin, I'd say being out 24/7 will definitely help. Bailey is a whole new horse now that he's on grass. I'd say work on dressage schooling to get the muscle tone(it really does help). Trot uphill and downhill if you can, it helps tone them up well too.
Proud mommy of...
Bailey- Appendix Gelding|Satin- Morgan Mare| Teka- Reg. Paint Mare|Shiloh- Reg. QH Mare| Misty- Reg. Pinto Mare