I think you look fine on your mare. And as an added bonus, the riding will help you get fitter. :) I lost 40lbs one summer just riding my APHA mare four times a week and eating better!
I think percentage ratios can be misleading unless you have hard, cold fact. I don't find weight tapes to be particularly accurate at all times, so unless you are putting your horse on a scale, I think it would be difficult to put a straight yes or no on something. The above APHA mare passed away (completely unrelated causes) and we had a necropsy completed and she came in about 300lbs heavier (and fit fit fit) than I expected her to. It's like with people - I might show you a picture and say "I weigh X lbs" and you could believe me but a scale could say an entirely different thing - weight looks different on every body.
Anyways... good on you!