John Lennon was one of the most prolific songwritters of the 20th century. Instant Karma is song with a message - the melody is secondary to the songs purpose. Its a song about love, about hypocrisy a song that takes ribs at Paul McCartney "Who do you think you are - a superstar? Well alright you are".
The production is instantly recognizable as Phil Spector's work. This was his comeback production after "retiring" in 1966.
The title is contradictory - karma is about the future (this life or the next). John uses that as a rhetorical device to essentially encourage peace, respect and love NOW - and about the rewards or costs or living otherwise. To John - Karma is something you reap the benefits of almost right away - or not.