"And I am willing to bet, the majority of the thousands of people in attendance that night in Philly, thought the same thing."

Even if they did, maybe they didn't mind.  From TicketMaster Fan Reviews:

"I went there to see Tedeschi Trucks and was blown away by all three bands! One of the best shows I have seen.
Favorite moment: I must admit it was a Grace Potter guitar solo. Dressed in her white bath robe alone in the middle of the stage. It was raw and powerful!"

I wonder if any other rock stars have worn a designer dress like that in concert. Kudos to Grace for doing what feels right for her.