That she has European features, you mean? Isn't she of mixed parentage as well? I thought it was canon that her father was African and her mother was... well, undetermined, since we never saw her, but some variety of American, right? So maybe she was white, and Vixen got her features but her daddy's skin color - or perhaps she was a Creole from New Orleans. They used to really emphasize the European nature of their heritage, so if she came from that sort of a background, it's not surprising her daughter has European features.