Probably should have blurred the edges of the cutie marks a little bit so they can match the lining.
Or probably used sharper lines. You can very much tell that the cutie marks are copy pasted images superimposed on an upper layer on their hips. The hair highlight is very sharp too in contrast with the lining layer. This looks like fill buckets were used quite a bit. The lines break too a little bit here and there throughout the animation.

Several elements in the image just not blending well and make it look messy a bit. If you tighten those up I think you'd have some really really nice looking work in the end.

My first recommendation is probably to use a brush that has sharper edges and doesn't fade like the one you used for the lining because you can see the different strokes made with it because of the blurring and fading.