Is it me or is this the first story (chronologically speaking, of course, not in terms of when they were made) that treats nudity as an everyday occurrence?

Yeah, it is. Though that might change. Body shame has never been a big issue for the Rogues, though, notwithstanding Jake's confusion at the end of "The Green Knight's Belt".

Or Kel at the Sabbath in Stone of Contention. Though that may have been more because of the stage fright.

Yeah, she was just feeling exposed there.