I do think Nidorina is kinda the odd cross between Nidoran and Nidoqueen.. Sort of the awkward teen years.. At least that is how I always saw her and Nidorino as well.
I disagree. I think it is half and half.
I personally think it looks ok like it is.
Even the main picture on http://bulbapedia.bulbagarden.net/wiki/Nidorina is showing Nidorina on her back feet.
I say let it roll with it.
that's just the pose they chose for the official artwork, she is supposed to be a quadroped and i think it would look better if it mirrors the nidorino model.
Nidorina is comparatively larger than her pre-evolution, though she loses the forehead horn, whiskers, and incisor teeth. She remains quadruped. Nidorina is also turquoise in coloration instead of bluish lavender, with a pale underside. Nidorina's poison spikes are larger and probably more potent; she has also gained the ability to stand on her hind legs.
I suppose both of you are correct.
all 4 pictures its on 4 legs, the art work just happens to be standing up, like what mice do sometimes, and if you look at the sprite in hg/ss when it walks behind you, its also on all 4s there