My Little Pony: Friendship is Magic - Image #648,952

If both of Luna's irises are near the back of her head, then if we looked at her facing straight on, her eyes would be derped in opposite directions. A more realistic way to depict this would be to have her left eye (the one we're seeing reflected in the mirror) be looking more towards her nose, to match the direction of the gaze of her right eye (the eye closest to us).