So very lovely!
I like how you made a dark background and have her be bright and light, standing out amongst everything else. The shading is great, her eyes are adorable!
Her mane and tail make her even more cuddly, too! It's a style I don't see too often besides on Pinkie Pie and a few other ponies. Many ponies have the wavy, smooth hair, but yours has a unique poofy sort of mane/tail, very good!
The only things are that she looks a tad bit on the skinny side, and the colors of the mane/tail and the body seem a bit too alike to each other.
Other than that, I could keep staring for days at this magnificent art.
I'd love to see her story, esp. behind that very nice cutie mark!