Please share a picture. If she's 35" and 250# she shouldn't be fat, that would be pretty light for 35" mini. Does she still have her winter hair? What are you currently feeding her? Have you run your hands over her? Can you easily feel spine, hips or ribs? Or do you need to press hard to feel them?
My 3 colts, as two year olds (last year) were 35-36" tall, the lightest was 230# and the heaviest was 325#, and none of them were fat. [They were weighed on a scale, but I don't remember if it was calibrated before weighing.]
This was May 2014:
Lightest and shortest boy:
Heaviest and tallest boy:
Middle weight, same height as heavy boy:
The heaviest boy was sold last spring, shortly after pics were taken; the other two I still have (they were gelded shortly after the pics were taken), and they are chubbier this spring, I haven't weighed them to see where we are this spring.