The drawing itself looks accomplished (well done!), but the background is distracting. It has the same amount of light as the foreground.
So what that results in is this: on the thumbnail, she looks so good, but in the actual art piece in full, I'm getting a bad headache.
Probably grey out (less saturation) on the BG a bit? Thinner, less bold lines? I know it's stained glass but the thing with thicker lines is, they help us define what is in the foreground of a flat image. Mash's outlines are so thin that she basically almost disappears into the BG.