If they get too warm they burn calories also. If she is shivering more hay should take care of that. As long as she has shelter and good hay you should be ok. When it is 25 here a mid weight is a bit warm. No way should you even consider a heavy weight. I don't normally blanket but it is 15 below today and I had hoped to be in Florida by now for the winter. North central and not sure if I will blanket at night there and feel my mid weight may be too heavy for Florida.
I live in NW Fl and just moved from a home that had a barn to one that had no fencing or barn. I fenced the property and brought the 3 horses over. My old guy is 22 and hard to keep weight up, but has 24/7 access to round bale as well as fed senior feed and Cool Calories 100 twice a day. He has not had a blanket on him and is doing fine even with the temp drop last night from 62* high to 38* low and going to be around 31* tonight. They have plenty of trees and brush to help block the cold wind, but no shelter at this time to get out of the rain. They are none the worse for being out in the open as they hardly went into the barn when at the other place. They would turn bum to the wind while under a tree and shake it off when it quit.
I know some locals who do blanket theirs and it is merely by choice as to whether you do or not down here. I do have plans on building a lean to with stalls this next year, but not a top priority.
I would just watch her. If she shivers and is obviously having a tough time with the cold/weather, then go ahead and purchase something lightweight for her. If not, then spend the munnies on hay.
I always let the horse dictate what they need. XD
A blanket impedes the horse's ability to fluff it's coat to trap heat. She needs access to hay 24/7 as the act of digestion generates core heat. I would recommend a weatherproof sheet if she's going to be exposed to cold weather along with cold wet wind.
Real coats just do so much better at keeping horses warm. Under that pile of snow the pony is warm and dry. I've seen 4 inches piled on her. Movement knocks the snow off rather than melting most times.
Up here I would have to have an entire wardrobe of blankets. This week alone the temp has gone from 45 degrees to subzero. It's rained, it's snowed, it has been windy and rainy. Now it's 5 degrees. I would have to have several different weights and backups for when the blanket got dirty or soaked through. The 3 horses are also entirely different sizes so I'd be looking at the very least 4 blankets x 3 but to do it right here 6 blankets for each. 3 different weights and duplicates for backup. I really hate seeing horses in wet blankets. Nothing more miserable.
I do have a mid weight stable blanket that I have put on the pony for a few minutes to defrost a good case of icicles from time to time. Her long curly coat will get them hanging near her belly from time to time but I haven't had to break it out in 3 yrs. After just 20 minutes she will be steaming when I take it off and giving me the stink eye.
When I got my walker she was maybe even a little thinner than this mare. Her back along the entire top line had the worst case of rain rot I had ever seen. She had bald patches the size of my hand in several places and lots and lots of little ones. It was the end of Oct. And it was already starting to spit snow. I did worry and watch. Attacked the rain rot as best I could in the cold and just kept the hay coming and kept adding more hard feed by the handful each day. I never once saw her shiver. The first week I was out there every couple hours because I was worried she was still alive not about whether she was shivering. I would have gotten her a heavy blanket had she shown any sign of being cold but it never happened. She does have a much finer coat than the other two but she's warm.
The OP's horse is many months away from riding. She doesn't need to be clipped. Clipping is to aid cooling down and grooming time when the winter coats are in. Last thing she needs is being clipped until she gains a substantial amount of weight. The money spent on the right blankets is better spent on hay.