The majority of the original villains used in the show after season one have green skin because they were all exposed to gamma radiation at one point.
The Hulk and the members of his rogues gallery who were mutated by gamma radiation like he is all have green skin, plus Spider-Man and the Hulk are both Marvel properties. It's dubious that this is just a coincidence. As for Mysterio having green skin and appearing out of costume in his final appearance, he may have heard about the other villains Spider-Man fought and deliberately exposed himself to gamma radiation to try and get himself an edge in fighting the wall-crawler, ditching his famous costume because he felt having green skin would make him unique enough already.
The characters live in an alternate universe where America was never diversified.
They lived in a world where africans were never brought into the USA. If you go back and watch the
Banned Episode 'one-eye idol', Betty's shocked reaction to the mysterious tribal warrior's appearance would make a bit more sense.
