Two year old horses will often go through a skinny and narrow stage. She just needs time to mature. I agree, give her all the grass hay or timothy hay she can eat. I would cut out the alfalfa is she's not eating her grass hay.
For her "feed", I would go with a small amount of oats (one 2qt feed scoop a day) with alfalfa pellets ( 2-3 2qt scoops a day) and a vitamin supplement like Select II or Equi-Base Grass (Uckele Equine Nutrition). You can add some fat in there for extra calories. Corn Oil is easiest, start with just 1/4 cup and work your way up to 1 cup a day. Stabilized Rice Bran works well too, as does Nutrena Empower (flax/rice bran nugget).
This combo is high in protein, high fat, low starch, and has all the nutrients she needs, along with the alfalfa she likes ;). Once she's at a good weight, cut out the fat supplement (oil or whatever you decide to use) and cut the oats & alfalfa in half. I agree that simple is better. I won't feed my horses any sweet feed or feel pellets. My hard keeper gained weight and kept it on with a LOT less food once I switched to a more natural diet.