Naomi is a girl seen as a background character. Her name was revealed in What's Your Favorite Word? which she was on when she was 10 years old. Her favorite word is "happiness".
Happiness Naomi is thus a natural nickname for her.
Interview for WYFW
"Happiness" is my favorite word, because you don't need a reason to be happy. You can just smile and be happy, and no one can ruin your day if you don't want them to. There's no limit for how happy you can be. I promise, if you just smile, you'll be happy!