I mean, she was excited to see him because of two years away, that's the same though for all of the crew.Usopp and Nami are the only 2 real humans on the ship. That's a great attraction to their characters. They're just 2 people using weapons, while everyone else on the ship is a monster in comparison. They both act scared, and that's why they're always together. Obviously nothing romantic between them, but the have the best friendship. Nami was also the most excited about seeing Usopp after 2 years. They connect in ways nobody else on the ship connects. Chopper is with them a lot too, but obviously this friendship thing is limited to 2 people only, so I can't say the scared trio has the best friendship.
Nami and Luffy's interactions based on a friendship point, even before you'd consider romance, I think are the strongest right there with Luffy and Zoro. Idk, I think they've got the strongest friendship besides Luffy and Zoro, based on what has been shown. Usopp has a better friendship with Chopper, imo.



















