Being a wee-bit goth-y my own self, I love the Pinkie's Boutique line of MLP toys- even though I have to imagine Rarity would be most distressed at a certain party pone opening rival boutique in the small hamlet of Ponyville.
I dunno. Rarity's boutique is definitively not the place for punk/emo clothing. Rarity's classy, so crazy fashion won't cut it. I bet Pinkie's store is full of either costumes or clothing that screams "LOOK, I'M TOTALLY OUT OF THIS WORLD!" Rarity wouldn't mind much the competition, but Twilight may get shocked if Pinkie's fashion becomes more popular, and Spike would take her line from episode one "Everypony in this town are CRAZY!"