It seems to me the only legal issue here is the fact the hat’s are being marketed with reference to the show. The simple design could never be copyrighted, and even if it where, any alteration to that design would make it unique and not covered under the copyright. So if the hat’s were to be sold as say “Brown Coat Apparel”, then they’re would be no legal connection to the show. And the hats would simply be considered similar to the one Jayne wore. The whole point is sell them as an associated garment, and anyone who’s a browncoat will know what they’re looking at.