Spider-Man debuted in “Amazing Fantasy” #15, published in 1962; he got a solo title, “The Amazing Spider-Man,” the following year. 1963 was also the debut of The Avengers (as a team, at least) in, well, “The Avengers” issue #1. As for the specific issues featured in “The Novocaine Mutiny”? They were both published in 1969. Long story short — neither comic in “M*A*S*H*” existed until more than a decade after the Korean War had ended.
Moreover, Marvel Comics wasn’t even Marvel Comics yet. The company existed, but from 1951 to 1957 (a timespan including the Korean War’s duration and end), it was known as Atlas Comics. It also wasn’t publishing superhero books at this time — those were largely verboten thanks to the moralistic crusade of child psychiatrist Dr. Fredric Wertham. Instead, the publisher specialized in monster and sci-fi books, taking the premises of B-movies and rendering them as eight-page stories in the funny books. Atlas Comics changed its name to Marvel Comics in 1961, which coincided with their reentry into publishing superhero stories.
By 1976 (When “The Novocaine Mutiny” first aired), Marvel Comics was fairly well-known (if still not the cultural juggernaut they are today). Grabbing a few spare issues as minor props would be easy, and it communicates to the 1970s audience that Radar is a nerd. It’s still anachronistic to the world of “M*A*S*H,” but it’s too minor an oversight to be immersion-breaking (the comics are barely visible). It’s also not a complete historical aberration.