I agree to all what rookie said, and yes, that is what I meant - if she was started at a young age and then worked hard she might be burned out on the riding idea. If this is not the case, then it's good, of course.
If she is just mellow by character, you won't be able to change it with training. Not all horses are born to be ropers, cutters, jumpers or dressage prospects, even if their bloodlines and conformation fits the deal. This mare sounds like she'd be a great confidence builder for kids or, let's say, a driving horse for hobby level driving.
I have come a long way, to surrender my shadow to the shadow of my horse.