Puzo could not resist inflicting us with bits of knowledge he had accumulated--whether or not the bits of knowledge had any relevance to the plot. Johnny was included in the novel to make a parallel with Frank Sinatra, on whom he's modeled. That part's relevant. Puzo did go on and on about Johnny's film and recording work, which added nothing to the plot but gave Puzo a chance to show off the knowledge he'd acquired about "show biz." The really boring stuff was his coverage of Nino (a completely forgettable character) and Jules and Lucy. Their affair seems to have been included solely so that Puzo could inflict on us the details of Lucy's gynecological operation--another bit of erudition he acquired somewhere and couldn't resist passing along.
Fortunately, these are minor blemishes in a great novel. But, if you thought that stuff was boring, wait till you read the Hollywood crap he loaded into "The Last Don."