I will say that I feel like there's an ... okay argument for Whitney, if only because having an actual type advantage at that point of the game would be hard.
That being said I would personally choose Ultra Necrozma or Cynthia over her on my Rushmore, if I were to use my fourth slot on a Pokemon character. Though thinking it over Cynthia makes WAY more sense to me. She's got that meme factor that Whitney has but I think she's altogether more challenging.