Growing up and living in the South I've known a lot of Carolines, including my own sister. I consider it pretty common, though not in a trendy way and not too common to use. It's a beautiful name.
I prefer no NN. My sister has always gone by just
Caroline, though within our family we sometimes call her
Lina (pronounced LIE-nah). Her name is mispronounced as
Carolyn quite a bit, and it's a pet peeve of hers.