Course she can do dressage. Almost any horse can get to medium, it's usually the flying changes that get them stuck. If you get some good lessons with a reputable dressage instructor and have sensitivity as a rider there's nothing stopping you from giving it a shot. If you're a junior rider, you could very well be competitive on your mare as well if you have to knowledge and skill to get her off the forehand and ride a nice, sot and accurate test.
I beat alot of super expensive wambloods as a junior on a 15h QHxappy mare who was build hugely downhill, was 12 when I started working with her (she had done nothing other than trail riding) and had the work ethic of a crazed grizzly bear but with decent training and a few sneaky 'faked' moments from me, she did quite well in prelim/novice. Elementary work was harder due to her conformation, she found the higher level of collection diffficult and just didn't have the brain to be a trainable horse.
Sorry for the long winded reply, but don't let anyone tell you that your mare can't do at least basic dressage and be competitive at lower, junior levels :)