Board Thread:General Discussion/@comment-46714970-20200824180548/@comment-38952966-20200824204234

The idea is that the Green Baby was a sort of... reincarnation of DIO? That's really the best term for it, even if it's not accurate. DIO possessed Jonathan's body when he gave Pucci the toe bone that created the Green Baby, so it also had the Joestar birthmark. When Pucci fuses with it, he got it too.

As to why Weather Report gets it... well, you've got me. Araki wanted to foreshadow that Pucci and Weather were connected but it doesn't actually make any sense as to why Weather would get the birthmark just because Pucci did.