To me the biggest factor here, is her appearance. It's not that Oda is incapable of making distinct female appearances, but more so that due to time constraints he sticks what he is good at. I feel like if his intentions were to make it so Yamato joins the crew, then he would have taken that extra time to diversify her face so she doesn't look like Nami, Rebecca, Vivi, Shirahoshi... etc...etc.. but with horns.
But who knows, maybe Oda just really liked the design, and wanted to keep it simplistic, and like
@Garp the Fist said, if he keeps the mask as part of her shtick, then it could work.