Yep, certainly looks like him, and yes, on closer inspection, it's a bouzouki, not a mandolin. The sound is murky, so can't confirm that's what he's playing.... It's a very brief shot while Dylan and "Sylvie" walk past (for those with access to "advance" copies of the film, it's at 1'40"18.)

Go see the film, Helen - it's a lot of fun. (So many historical inaccuracies, but it's a film, not a doco. "Sylvie" wasn't at Newport '65, neither was Johnny Cash, and I really don't think Bob rode there from NYC on his Triumph, for a start....) Chalamet is uncanny and extraordinary, and the rest of the cast is terrific and well-played, except for Grossman, who is portrayed as an over-blown cartoon-character.