Why was she on punk mode at the trial ?

The book explains that she did so in order to present an enhanced version of the image everyone had of her, coming to court as herself and no-one else. Dressing up in formal clothes would mean conforming to traditional expectations and would seem out of character for her, as if she were trying to present a false image to the judges in pretending to be someone she was not. It would in some sense mean a small victory for the prosecutor and his case.