That might be true. I think the reason she´s a tomboy is because most of the ’girly´ girls are raised in safe and richer environments, they have time and money to provide themselves good looks and do things for fun.
Vitani and Nuka both look skinny, are always beeing trained and pushed around in order to survive. I think that´s the reason she´s boy-ish: She has no father to protect her, Her older brother is a bit of a retard, so she has to stand up for herself and protect her own a..e.

That´s, in my opionion, the reason she acts so manly
And for that reason, I like her more than the safely raised and protected princess Kiara.
(no I don´t dislike Kiara, I just like Vitani more)