I wouldn't worry about her looking immature.
Yeah, perhaps a higher protein diet (I would e looking to adjust grain types and amounts if she gets a lot of grass and hay already) but really she looks to me to be more gangly and awkward than serverly under weight.
I would however be looking into getting grain upped.
What type of grain is she getting? I find pellet feed seems to work better for me than sweet feeds. I personally like Purina Ultium or Purina Strategy myself... But those are easy for me to get in my area.
I think more than anything she's just a slow growing gal that needs a little more time to fill in and grow up.
Every year while apprenticing with a local trainer we got at least one two year old (sometimes a three year old) that we would break enough to sit on and walk around the she would send them home till next year telling the owners to give the horse another year.