Rainbow's posing, compilation, expression and lighting is perfect, however, the rest of the scene could use some fixing up.
The moon and stars should be in focus, as well as rainbow, even though it feels illogical. I feel as though being able to clearly see what Rainbow Dash is seeing would add to the scene. If you search for night time landscapes you'll find that at every point the moon and stars are in focus. This is because they are main elements in the scene. I personally suggest removing the blur entirely.
The main question to consider is "What are the main elements of the picture?"
In this case, the main elements are Rwinbow Dash and Rainbow Dash's focus, the moon. As such, both should be in focus, and since there really isn't any logic in making the sky not blurry, but the horison blurry, the best solution is to remove the blur.
One more suggestion I'll have to give is the fact that the grass shouldn't be so orderly, grass is, by its nature, disorganised, and those lines you can see there are more suited for farms and crops. If you wan to keep the organised feel, you can replace the grass with Appletrees or even wheat.
The last suggestion for this picture is for the elements in the horison, since the horison is not a main element of the scene, this isn't really important, but I think it could look more lively if you added some mountains, or continued the trees across. Either way, this is not that important, and it can be left alone.
Another point to consider is the Rule of Thirds, as any photographer would tell you, the Rule of Thirds is an integral part of any and all photography or rendering.
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