It's even more juvenile than Danny or Charlie. It's very stereotypical, so much so that there is a genre of jokes, "Little Johnny jokes." Featuring a smart-alecky little kid named Johnny. It's kind of like the old Mutt and Jeff genre.
Johnny on a grown man seems childish as a nn, and very redneck if it's the name on his birth certificate, as if his parents couldn't even be bothered. It's almost as bad as Junior or Buddy or Buster as the full name.