I really like what you did with Luna's posture as well as the colors!
I also like the simple and minimalistic Moon, especially for a shirt design.
However, I feel like the two clash with each other. The amount of detail on Luna makes me expect the same detail and depth from the background, and the background's minimalist style makes the foreground (Luna) "distract" my eye rather than please it.
Just wanted to throw in some constructive feedback, which may or may not be complete rubbish since I know nothing about producing visual arts. =S
Anyway, lovely image overall, and a big brohoof! ^^/)
I cannot even begin to explain how much I am absilutely loving this piece! The character desing and environmental work is simply brilliant and resonates so well with the overall emotion of this piece. Your use of colors, lighting, and shading is just magnificent and the nonchalance of the piece creates a beautiful sense of peace and light! I really love your style and how you implemented every element within this piece! I am extremely happy to add this to my favorites and I very much hope to see more! Great work! Kind regards, Princess Luna