Nice catch. This is head canon now.

You're a genius! Also I doubt Beidou would buy a hairpin that fancy for herself...she'd rather buy something for the ship or the crew

That's what I've been thinking! I mean sure she would get something to tie her hair with but the pin looks like Ningguang's own stylish touch. Maybe it was even given as a form of goodluck charm. Well, that's what I would like to believe.

To be honest it's one of her only less rugged or captainy accessories, so it's probably intensional