No.

Every big city in Italy has pickpockets. Napoli is nowhere near as dangerous as any major American city. Tourists get ripped off everywhere. Beijing is one of the safest cities in the world and you had better believe I've been ripped off there (though much less as time went on as I lived there).

Napoli is worth more than a day. The museums, markets, pizza (l'antica da michele and di matteo come to mind) should be more than enough.

And if you have not been give the ruins of Pompeii at least half a day. An easy and short train ride from Napoli.