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.