In the original GotG special features it was said that Karen Gillan shaved her own hair before showing up to play Nebula. Like any long enough running character in comics Nebula had dozens of looks to choose from and also the flexibility that the MCU isn’t always strict about matching comics appearances anyway. They all said Karen binged the relevant comics and wanted to make it clear which iconic version of Nebula she wanted to play, and how. It’s great she got a full arc to go with that willingness to put in the extra work.