I'm sorry, I.know you're young a d can't get her proper veterinary care, but yes, the
black beard indicates she's either in pain or very stressed.
I agree completely with Tracie, this looks like a total lack of
UVB light, resulting in a severe Calcium deficiency and MBD. SolarGlow MVB lights are not very strong at all, and usually test well below adequate for a bearded dragon. In addition, they are only good for 10 months or so, after that they stop emitting any UVB or UVA light at all, at that point they only emit heat. So if you got it second hand and the writing is worn off, there's no telling how old it is, it probably hasn't emitted any UVB or UVA light for a long time.
Without any UVB or UVA light she cannot properly digest, absorb, or process/use any nutrition she's taking in from any food or supplements she's eating at all, including Calcium and all other vitamins or minerals. Plus, the SolarGlo is not a strong enough MVB to use on top of any type of mesh tank lid, they block 40% of the
UVB light, and the light is too weak to begin with.
I suggest either ordering her a proper long (at least 22") T5 strength UVB tube and a matching T5-rated tube fixture on Amazon.com (cheapest place by far, you'll spend right around $50 shipped for both) and then removing the SolarGlo MVB from whatever fixture you have it in and then getting a regular Halogen indoor flood bulb from Lowes, Tractor Supply, or Home Depot, probably 100 watts if her tank is a 40-gallon Breeder Tank, if smaller then go down in wattage to a 75 watt Halogen indoor flood bulb (they only cost between $5-$10 at most, Tractor Supply sells 2-packs for $9), and using 2 separate lights from now on, the long tube for her UVB/UVA light and then the regular, halogen indoor flood bulb for her heat. Both lights need to be on for at least 14 hours every single day.
The 22" Reptisun 10.0 T5HO UVB tube is the UVB tube I'm speaking of on Amazon.com, it costs $23 if ordered there, and it's strong enough to sit on top of the mesh lid, and only needs to be within at least 11" of her Basking Spot/Platform. Also, the T5 UVB tubes only need to be replaced once every year. Then a matching 24" tube fixture that is rated for a T5 strength UVB tube and includes a metal reflector inside it costs about $28 as a package deal on Amazon.com, so around $50 shipped for both, which is by far the best deal you'll find anywhere.
Just an FYI, you cannot use any compact or coil UVB bulbs for a bearded dragon, they are all far too weak, and if obstructed by a mesh lid they will not emit any
UVB light to her at all. They also cause eye and neurological issues, and this might also be part of her current problem, as an old SolarGlo MVB that is only emitting plain light may be causing her distress at this point too. Also, most all of the UVB tubes that you'll find in most pet shops (any Petco or PetSmart stores for sure) are only the much weaker T8 strength UVB tubes, which are too weak to help her at this point. Also, they are much more expensive than buying the proper T5 strength UVB tube and fixture on Amazon anyway. The T8 UVB tubes cannot sit on top of the mesh lid, must be replaced every 6 months (so you save money that way too if you buy the much stronger T5 UVB tube), and must be mounted not only underneath the mesh lid but must also be no further away from the basking spot than 6". They aren't worth the hassle nor the extra cost, that's why I recommend ordering the 22" Reptisun 10.0 T5HO for $23...In comparison, the shorter, weaker 18" Reptisun 10.0 T8 UVB tube at any Petco or PetSmart costs $37.99 for only the UVB tube without a fixture!!!
Also, just to make sure you don't end up buying her a brand of UVB tube that will make her much worse, please avoid the
UVB light brands All Living Things (PetSmart's house-trained), Zilla (Petco's house-trained), and ReptileOne. These all are manufactured in the same Chinese factory and as a byproduct of the manufacturing process, they all emit harmful light rays that cause eye damage, blindness, and neurological issues, in addition to being far too weak. At the point she is at she needs a very strong, intense, high-quality T5 strength UVB tube 14 hours a day to try to start reversing the damage. Without this immediately, she's going to lose her appetite completely, stop moving, and most likely pass away. But I don't think that she's a lost cause at all, and do not suggest just euthanizing her.
By the way, please do not try to euthanize her at home, when I read your question "Is there a humane way to put her out of her misery" I got sick to my stomach, the answer is no. First of all all she needs is a T5 UVB tube and a fixture for it, around $50 or so shipped from Amazon.com and I think she'll start recovering very quickly, along with force feeding fluids, nutrition like slurries made from healthy greens and veggies and that include a pinch of Calcium and Multivitamin powders, and 3 proper temperature zones inside her tank. So euthanizing her is not necessary yet. Secondly, if it comes to that then you must take her to a vet to have her humanely put to sleep, you cannot do it at home. Anything you could do at home would be painful and traumatic for both you and her.
The last option here is to surrender her to a reputable reptile rescue, and this is what you'll need to do if your mom is unwilling to order a proper T5 UVB tube and fixture for her, along with a 75-100 watt bright white basking bulb or halogen indoor flood bulb to put right alongside the T5 UVB tube. I have worked at a Certified Reptile and Bird Rescue in Pennsylvania for many years, and if you are unable to get her the proper equipment she needs to get well, I can certainly find the closest, reputable reptile rescue to you if I know where you live. We have a very large, strong network across the country, and I'm more than willing to locate the rescue for you and get them to save their required donation for surrender for you to take her there. They can not only get her the proper lighting, equipment, and dietary requirements she needs to get well, but they would also take her to their Reptile Specialist Vet immediately for care as well. I know that would be a hard decision, but without the things she needs ASAP she's going to die, so obviously surrendering her is the better option. I'm just throwing that out there for you just in case.