^Nina starts off using her middle fingers in the series, but then decides on using her pinkies instead since she doesn't want to do the full blown middle finger gesture. An older guy basically says the same thing of is this what the kids are doing? when he's confused by it, it's whole funny thing.
I also get the feeling that age ratings say you get X amount of middle finger usage, and so they had to do an alternate gesture. As age ratings are wildly specific with what you can do and how much.
Regardless it works fine imo and is a fun part of her character