Oh goodness. Yes, she could definitely stand to slim down xD it will probably make her feel much better as well. Right now she's running the risk of cushings disease or something similar. I do realize though, that she isnt QUITE as heavy as she looks, because of her winter wooleys. Even in her summer photo she appears to be in need of losing a good 25 pounds. Don't you just love have 'too easy' of a keeper? XD
This is my mare's summer weight, approximately 225 lbs and 34" tall. This was when she was at her fittest just being let down into 'pregnancy mode.' I considered her to even be a smidge bit too heavy at this point.
Generally speaking a miniature horse (not shetland) should be trim and fit looking, with no excess belly or flank, and a graceful neck. Depending on their build some may be a bit heftier (your mare looks like she might be) but they still shouldn't have 'apple butts' or large, cresty necks.
This is our mini mule who is overweight and displaying the same belly that your mare presents. She could probably stand to loose 15 pounds or so.
This is probably the optimal weight for your mare. Our little gelding looks to be built similarly to yours (though butt high) and is at his ideal weight in this picture. Streamline without a bulging belly, but by no means thin. He was in pasture condition just as your girl is in the photo.
Hopefully that helped! Your mare is VERY cute.