I've always suspected it, but now I've confirmed it. Spider-Man is a Mets fan. Back in the 1970s, Marvel Comics teamed up with The Electric Company to make easy-reading Spider-Man comic books for kids 6-10 years old. They also aired versions of these comic book stories on episodes of The Electric Company. One such story (and episode) is "Spidey Up Against The Wall", where Spider-Man, in full costume, goes to a Mets game at Shea Stadium with a Mets hat on, yelling "Let's Go Mets!" You can see the episode here. It features a ridiculously awful villain and a hilarious appearance by a young Morgan Freeman as an umpire.