I always thought that the picturesque was a rather arch comment from elizabeth, as in my understanding this term was mainly applied to arrangements of nature, ruins, and animals - but not people. I may be wrong. However, given my understanding, you realise why I always thought of that comment rather as a private joke, highlighting also the lack of (art) understanding of the sisters and why I'd expect Darcy to chuckle with her comment admiring her wit, education, and teasing, rather than praise her grace.