Question: Why is the Princess called Peach instead of Toadstool and why is the main villain called Bowser instead of King Koopa?
Question: How old is Mayday supposed to be in this movie? It's supposed to take place 16 months (one year and four months) after Spider-Man: Into the Spider-Verse, which makes no sense. Peter would've had to re-marry MJ in his universe just like that and nine months later, their daughter is born, so Mayday is three months old?
Answer: Who's saying they got married first? They reconciled, she got pregnant at some point, and they got married whenever. Do we even know if they are married? Mayday could be as old as 7 months. She's crawling and seems to have basic motor functions like pulling her hat down, so she's likely at the older end of that scale. While she's big-ish, she's not talking or anything, so she can't be that old. Plenty of younger babies still have a lot of hair, plus we can't be 100% sure how his universe works, in terms of child development or indeed be sure that time passes at exactly the same rate as Miles'.
Okay, you made a fair point on the whole marriage thing.
Answer: She is an infant; she appears to be around 3 or 4 months old.
Answer: "Princess Toadstool" was used in the original English-language manual, but she was Peach in the original Japanese, and that name continued in later versions, being combined with Toadstool. Bowser was originally called "Kuppa", but presumably the English-language version resonated more with people, plus removed any confusion of "Kuppa, King of the Koopas", so the English version stuck.