It's false scandal-mongering aimed at undermining the authority of the Pope.
The myth comes from various sources: The name possibly comes from a Pope John, who was derided for being affeminate who may have been mocked as "Pope Joan." That probably accounts for the name.
There was an incident of one of the Medici family (Catherine?) giving birth during a papal procession. This woman was powerful enough, it was joked that she was the real Pope, like people nowadays scornfully refer to President Hilllary. That probably accounts for many of the details.
And then there the street named for a Mrs. Pape, which non-Italians could easily mistake for being named after a female Pope.
Just as intriguing as The Da Vinci Code
-- and just as phony!
Since the pope only has "authority" over Catholics, what do you care ehat non-Catholics think? If a silly legend about an alleged female Pope 1100 years ago can undermine the current Pope's "authority" then I'd start looking for another religion quick.