Hum.
Well, my first thought was - depends on her build. I was a skinny malink when I was that age too and my Dad used to joke that you could play the xylophone on my ribs. DS1 is the same. Long thin ectomorph body type (if you're interested, here is a link which is saner than many www.precisionnutrition.com/all-about-body-type-eating )
But I used to eat quite well, and still retained that level of skinniness into my 20s, as does DS1 (well, he's only just a teen so I don't know how long he'll stay skinny yet!)
DS2 on the other hand is not an ectomorph and has always had a bit more chubbiness on him. DS1 was 10th Centile for aages, DS2 hit 99th centile within 5 weeks of birth (for weight, I mean).
My next thought, when you said about her eating habits, was to wonder if she has any other spectrum condition traits - a friend of mine's oldest child was always very thin and lithe, but full of energy - diagnosed with ADHD at 7 and autism at 9. Not typical, "mild" symptoms, if you like, but lots of processing and sensory issues, including food.
If that's not an option then you're back to "picky eater" - but if she's still got plenty of energy, doesn't get ill and seems bright and well in herself then I don't think you need to worry about it. If her skin and hair were dull and dry looking, she has bags under her eyes, is tired and lethargic etc. then maybe you DO need to worry she's undereating - but if she's healthy but skinny, then probably not.