I consider her a tomboy too
cause I'm a tomboy, Vitani's a tomboy, and there has to be another out there
but technically the way their underfur is doesn't determine gender. I think Disney just didn't realize what a tendency they had to do all their lionesses with underfur that extended onto the neck, sometimes connecting with the jaw coloration on some, and did Tama the way they wanted. plus I think they based Tama off an old clip of a random female cub on TLK2 Active Play game, and that cub's fur ends at the chest.