I've not seen this one before. So here are my first thoughts. Immediately my hunch is that it is part of the photos that were part of Zink Magazine. My other gut reaction was Kermit is Howard Hughes in this picture because it looks like he has a pencil moustache and a scarf, both associated with Hughes. Andrew would be super good to get in on this conversation as well, but for now that is all I got. I did a reverse image search and came up empty handed, as did a google image search for "kermit the frog and howard hughes"
I don't know who designed that dress. I'm guess it falls under the costume designer for the film, although I'm not holding my breath on that. Do you have a picture of the PVC? Were there others? I wish we had something like that in the states. My movie theatres here didn't even have a drink cup like they do for Twilight or Alvin and the Chipmunks, which made me super sad.
You're probably right with that it's just a "generic" Piggy dress. Still, I present you the Muppets-Menü, showcasing the glorious PVC of her! I really like it lot, and was wondering the same about US movie theaters missing out on offering a cup or a toy or something to go with the film. Even the UK has those bobble heads (and the Miss Piggy one seems to have the same body as this one).