Pauline is BABY MARIO, as a trans 13 year old

Baby Mario is Mario's baby, NOT Mario as a BABY

Mario & Luigi Partners in time is not part of the main timeline, but also it is the only instance of us seeing younger Mario. Baby Mario just looks like Mario as a Baby because they share the same genetics / artistic design choices ala Bowser / Baby Bowser / Bowser Jr.

Yoshi Island is the plot of the stork delivering a baby to MARIO, which makes sense because Mario was born in Brooklyn, and Baby Mario is clearly born in the Mushroom Kingdom.

13 years pass, and Baby Mario decides they was to be called Pauline like their hero, the mayor of NDC