This is quite an amazing vector! There are so many little details that you notice as you keep staring at it, and the craftsmanship gets creepier and even more awesome the longer you stare.
Obviously, this riff on pony fashion has been done before by none other than Miss Pinkamina Diane Pie in her role in "Cupcakes" But that ensemble was patched together out of random ponies from across Ponyville. Twilight has clearly gone mad, and instead of taking the random approach, she slays all who are close to her... and Derpy. This piece hits home a little harder due to it just being the mane six, the CMC, Derpy, Spike, and is that... Nyx?
My favorite moment was when I noticed, "Oh my lord, the stitching is done WITH THEIR HAIR. That is amazing." The "saddle" the wings sit on is from Spike's hide, and seems to wrap around with a piece of Derpy's pelt to hang the horns. Everything seems to fit together so well. I also love how the raised hoof is done as a complete stocking, the asymmetry adds to the chaos.
Everyone will react differently to having an OC in the mix, but honestly the dark bit of, ahem "fabric" was needed to complete the visual. It contrasts and draws the eye, plus the character is so popular that it will hit the lovers hard, and it will please the haters to see her skinned, right? Right?
All in all, you've created a wonderful piece! I will have to peruse the rest of your gallery...
Going through My Deviant watch messages, deleting them all and was like Huh, wait what? Seeing has I have grown a bit tired of seeing Miss Pinkie in her 'party dress', I almost completely over looked this, until i took the second glance, notice. Wait that wasnt all pink?
Looking at it I was in complete Wow mode. The fact that you did use the main cast instead of just randoms Is in my opinion a great twist, even Twilight's beloved Spike. And doing the stitching to match the color of the ponies hair was a wonderful detail, that I think adds a great deal to this, even though it is so subtle.
To be honest, if Rarity replaced Pinkamena in 'Cupcakes', this is exactly what she would wear, but she'd have different cutie marks and she'd add a hairpiece made from bones or alicorn horns or something.
Oh and I noticed a small detail; the little saddle part thing is made from SPIKE'S SKIN!
Yo Twily! Might wanna go easy on the pills. Yeah, an' y'all might wanna slow down with the adrenaline shots. This coming from BOTH the adrenaline junkie AND the pill popper! ... *uses her magic to create a bottle of pills and an adrenaline shot* Hey! Look over there! *turn around and spot the pills and adrenaline.* PILLZ HERE! Adrenaline shot here! Go on boys, you know you want them... ... Grabbin' PEELZ. Grabbin' a shot! *Runs over and hugs Twilight.* I LOVE horses!