MTG might help with her mane and tail, once you get her nutritional needs sorted. Washing will only make it all worse, and that dries out the natural oils needed to promote a healthy coat and skin. Generally, my horses get only one or two baths a year (I do sponge off sweat), but they are groomed daily and their food is carefully monitored. Most pasture is simply not adequate to meet the nutritional needs of any horse without some sort of supplement, be it grain, minerals or whatever.
Pheets is always right, listen to her!
It is never the horse's fault