Yes, there's a parallelism problem. However, in informal speech and writing it's often acceptable to elide understood terms, and parallelism failures are also often ignored to simplify phrasing. Since only contestants can win prizes, it's clear that the last item refers to the contestant wearing the most interesting garment, not the garment itself. The other items also don't explicitly mention the contestant, but it's obvious that "best dressed" is short for "best dressed contestant".
If everything were to be said explicitly, it would be something like:
Prizes will be given to the best dressed contestant, the worst dressed contestant, and the one with the most "interesting" garment.
This is unnecessarily wordy.