Dahlia is quite an unusual character given her age, role and the genre of Skyland. She is, to some extent at least, the spunky, strong female character a lot of shows with this target audience have. But rather than reckless, impulsive, and fun-loving, she's super strict, determined, single minded, practical, and almost humourless (yes, she makes the occasional joke, but she's a long way from 'Comic Relief' status.)
Did something happen in her (completely unknown) past that led to this? She was 7 when Marcus disappeared and the Rebellion crumbled, would she (or her parent/s) have been a rebel then? Did the Sphere do something to her family, or a friend that led to her becoming a pirate?
I think it's pretty safe to assume she's not one of the foremost pirates simply because the Sphere's nasty and flying's fun. She certainly doesn't take her job (or anything, really) lightly.
Hopefully we'll get to see an episode focusing on Dahlia's, or the pirates in general, past (Cheng got one. ), but in the mean time, speculation is fun.