WHO assistant director Ranieri Guerra condemned the crowds amid the social distancing rules put in place to prevent the spread of the coronavirus.
"I wouldn't like that to happen again," Guerra told Italian TV programme Agora. "Seeing those images hurts."
Napoli defeated Juventus 4-2 on penalties to claim the Cup title in a match that was played at Rome's Estadio Olimpico with no fans in attendance.
"The parties in the [streets] are wretched, doomed to create huge problems," Guerra added.
Guerra also compared the celebrations to the Champions League match in Milan between Atalanta and Valencia in February, which is thought to have had a major role in spreading the virus in Italy early in the pandemic.
Napoli had not won a trophy since 2014 when they won both the Coppa Italia and Super Cup.