It is safe - but the wy is absolutly boring. Take a taxi or go by tramway. If you want to see the ancient city of Naples, you can walk. It is as safe as in other big towns. But beware of pickpockets at the main station.
We stayed in Naples two times for one weak each, and we came back with all our things.
Have a lot of fun in Naples
Uwe
The tram station (and the bus stop too) ist just in front of the cruise terminal (stazione marittima) at Piazza Municipio (Castel Nuovo). Nearly all lines go to the main station (Stazione Centrale; Piazza Garibaldi). It takes about 15 to 20 minutes. Beware of pickpockets. But it is as safe as in any other big town. We never had problems with that.
Ask at the information desk in the cruise terminal - or look on the display in front of the tram or bus.
The fee is about 80 (Euro-)Cent for one ticket (valued for 90 minutes).
Regards
Uwe