It would seem that it has been confirmed that Vivian is trans
You can read more hereNintendo GameCube classic Paper Mario: The Thousand-Year Door had its remake released on Switch this week, and it has confirmed that the character Vivian is canonically trans.
In the original's Japanese version, as well as translations for certain European regions, the character was always trans, or at least gender non-conforming. Her sisters in the group the Shadow Sirens bullied her for it, saying that she wasn't like them, which led to her allying with Mario later on in the game.
In the English GameCube localisation, however, this was censored and no reference is made to Vivian being trans. Instead, her sisters bully her for being ugly.