Oh I'm sorry, she does look very small for her age, actually a bit thin...Can you possibly take a photo of her enclosure that shows her lights and how they're set up? I think she has 2 different, separate issues going on here, one being a calcium deficiency and probably the start of Metabolic Bone Disease (MBD) due to inadequate UVB/UVA lighting (this makes her unable to absorb any of the calcium from her food or the powder), and then there is the yellow skin issue. What exact
UVB light do you have (brand, wattage or strength, and then type, meaning is it a long tube, a compact bulb, a coil/spiral bulb, or is it a Mercury Vapor Bulb)? How old is the
UVB light? And how far away in inches is the
UVB light from her basking spot/platform?
As far as the yellow skin goes, it does look like Yellow Fungus in that it's all over her and the color looks to be Yellow Fungus, but I don't see any "scabbyness" or rough, dry skin where the yellow color is, usually with Yellow Fungus the skin is that same yellow color, but it looks like scabs. Has she shed recently? It could just be retained shed that didn't come off, and actually sometimes inappropriate or too-weak of a UVB/UVA light and improper temps can cause shedding issues too, so I'm going to wait for others to chime in on their opinions of the Yellow Skin. Have you tried
bathing her and using a soft toothbrush to see if the yellow skin comes off? If not, you could certainly give that a try, be very gentle, let her soak for 5-10 minutes in the warm water (make it a bit warmer than usual but not hot), and then use a soft toothbrush to gently try to brush the yellow area off. Just try it on one spot and see what happens, if it doesn't come off at all then no need to stress her out by scrubbing all the other areas, just something to try because retained shed will usually come off. If it doesn't then you need to find a very experienced Reptile Specialist Vet, not an "exotics" vet, they tend to make things much worse. She needs to have a "Skin Scraping" test done, which will have to be sent out, this will confirm or rule-out Yellow Fungus, it's the only way to know for sure.
If she does have Yellow Fungus, it's very, very tough to treat because it grows both inside and externally, and only by giving them an oral Anti-Fungal medication long-term in conjunction with an external Anti-Fungal treatment will give her a chance. That's why I said not an "exotic" vet, but a very experienced Reptile Specialist that has treated Yellow Fungus in Bearded Dragons before.
Either way, she definitely is suffering from a Calcium Deficiency and MBD due to inadequate UVB lighting, or UVb lighting that is obstructed, too far away, or too old. Once we know what light you have and see your set-up we can definitely help you with that particular issue and get her appetite and energy levels back up, it's going to be extremely important to get this issue worked out immediately, because if she does have Yellow Fungus she needs to be as healthy as possible otherwise to have a chance of fighting the infection.