The 30-year-old took his tally to 36 goals, equalling the record set by Gonzalo Higuain for Napoli in the 2015-16 season. He also finished the season five goals clear of the next leading scorer, Cristiano Ronaldo of Juventus.
Napoli regained the lead from a penalty in the 54th minute, Insigne sending Strakosha the wrong way after Dries Mertens was caught by Marco Parolo when he was about to shoot. Matteo Politano added a third in stoppage time.
Lazio finished fourth with 78 points and Coppa Italia winners Napoli ended the season seventh with 62 points.