John once said that Paul wrote songs like MSH "for the grannies to dig." The funny thing, as I was discovering the Beatles, my maternal grandmother walked by when "Maxwell" was playing. She singled it out for praise: "It's got a nice beat."
George hated MSH too, as I recall, deeming it "really fruity."
If I were George Martin, I'd have urged them to leave both Maxwell and "Mean Mister Mustard" off the album.