Clearly Lennon held belligerent feelings towards the avant-garde. This was in part due to Paul McCartney’s status as the Beatles' avant-gardist - that was Paul’s territory, and John knew that, and could not impede upon it. And yet, despite his harsh words, he found the freedom of expression opened through avant-garde aesthetics and experimentation appealing. Quoting McCartney: “I was into a lot of those things, which was very strange because I was at the same time known as the cute Beatle, the ballad Beatle ... John was the cynical one, the wise Beatle, the intellectual. In fact at that time it was wildly in reverse. John would be coming in from Weybridge; he’d sit and he used to tell me he was jealous: ‘God, man, I’m so fucking jealous!’ He had to break free - which is what he did, later” (Miles p. 220). So Lennon did have some degree of avant-garde interest, but his embrace of the movement had to wait until Yoko provided the change he needed.
Blaney, John. John Lennon: Listen to this Book. Paper Jukebox, Biddles Ltd, Guildford, UK, 2005.
Coleman, Ray. Lennon. McGraw-Hill Book Company, New York, NY, 1984.
Miles, Barry. Paul McCartney: Many Years from Now. Henry Holt and Company, New York, NY, 1997.
Wenner, Jann. Lennon Remembers: The Rolling Stone Interviews. Straight Arrow Books, San Francisco, CA, 1971.