Yes, there was fakie ponywear. Quite a lot of it too. A few different companies got in on it. That shirt is from one of the main fakie ponywear makers. Their outfits came with stickers that used bootleg copies of real pony art:
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loginYou can see the outfit the plaid shirt goes to in the bottom right. Surprisingly, as far as I can tell, the outfit did not actually have the hat. The rest of the stickers seem to be pretty accurate as to what you got with each outfit. There's also a queen outfit very much like the MLP Pony Royal not pictured(ignore the German pony club sticker:p). I have the sticker, but it didn't make the photo shoot:p
It's close enough to the real thing that it's confused more than one collector as to whether or not it was legitimate Hasbro ponywear. It's not. It's fakie wear designed to go on MLP. It's probably the most common of the fakie sets out there, and it's also designed with MLP in mind. It fits the best of any of the fakie outfits, and it's pretty decent quality all things considered.