The poet wears clothes on stage. If, at any point, those clothes are altered, referred to, etc, that makes those clothes a costume. It's the reason that a tattoo can never be a prop - it can't be taken off. It's a given, as it were.
Also, a poet can be disqualified if it's decided that removing the clothing, using a costume, etc, enhances the performance. Otherwise, if it was unintentional and doesn't enhance the performance, it's a 2 point deduction.