(( Disclaimer: I roleplay as Rarity, and I'm going to let her say what she wants about this piece. ))
On the left, I'm assuming that she's simply using the measuring tape as a prop, and not actually measuring the dress, as it looks to be finished, even including a belt on the mannequin. I appreciate the color scheme for the outfit, having it in purple, white, and just a hint of black. I also like the fact that a corset is implied but not actually used in the outfit, as it's a clever reference to how fashion changes at a moment's notice. Now for the right. I'm reminded of "Suited for Success" when Rarity was talking to the camera and made the duckface ( for reference ) near the beginning of the episode. However, I have to agree that the Cutie Mark is slightly misplaced, as it should be about 4 and a half inches higher on the buttock, as that would correspond more closely with where it is on the actual pony.
Overall, I have to say that this was quite well done.
(( I have to agree with Rarity here; after all, my personality is a lot like her, even when I'm NOT roleplaying. I suppose that's why I chose her to roleplay as. ))