Tomboys would probably have their hair short... as stereotypical as that sounds. Additionally, as she’s a mechanic she probably wouldn’t want to have long hair get in the way of her work. Or she couldn’t be bothered with trying to keep it up (washing, blow drying, hair products, haircuts etc etc). Basically, short hair is a lot easier to take care of quickly and with less effort than long hair.
Edit: You may not know what a tomboy is, it’s a somewhat older word, probably is not considered politically correct from some group or something. But here’s the definition:
a girl who enjoys rough, noisy activities traditionally associated with boys.