Is Gene being realistic when he blames Finny for deliberately runing his grades with all his diversions?

No, Finny did not deliberately sabotage Gene's grades. In fact, Finny naively thought that Gene was naturally good at school work, like Finny is at sports, and never needed to study to get good grades. Finny insists that Gene needs to study if he thinks he does; but Gene decides to defy Finny's advice, and, against his better judgment, goes to the jumping tree with Finny.