I would say that as a Beatle, John Lennon was a very good songwriter, with moments of brilliance (She's Leaving Home comes to mind.) But a genius? Nope. I believe Paul McCartney was the better songwriter. His songs seem to have a longer, stronger shelf life. What song does Lennon have to match Let it Be, or Hey Jude, or the Long and Winding Road? Or hell, even Birthday? But would you say McCartney was a genius? JS Bach? Hell no. A very, very good pop songwriter. Lennon? At times a good songwriter.
Lennon and Mccartney were songwriting geniuses. There are different levels of genius obviously. Willam Faukner was a genius but not on a level with Shakespeare and Milton. ‘She’s Leaving Home’ was Mccartney btw. Birthday was both of them. Let it Be and The Long and Winding Road aren’t very good songs!