I'm looking at flights from US to Italy. Specifically, EWR to either Rome or Naples. Tap Air Portugal is one of the common best results and I'm wondering - is a 1:40 layover too short to get through EU customers there? This flight gets me to my destination quicker than the next option which is a 3:30 layover. But someone told me to make sure I have enough time to get through EU customers in Lisbon.

One ticket or two?? That makes a significant difference. If it is on one booking/passenger #, you will go through Immigration (border control/passport check) at Lisbon but your luggage will be checked through and you wouldn't see it til landing in Italy, where you will do the Customs bit (Customs is really a non issue, its Immigration that can take a lot of time)

IF it is on two tickets, your luggage wouldn't be checked through, so you'd have to collect it at Lisbon. You'd do both Immigration and Customs at Lisbon then have to go back through security to board the next flight.

If it is on one ticket you are 'protected' which means if you do miss the connection, they will put you on the next available flight -- two separate tix -- you are on your own and would have to buy a new ticket.

One ticket - I'd feel fine w/ a 1:40 connection. Two tickets - no way.

If both legs are on TAP then you could (with very high likelihood) check your bags all the way through to Italy so you wouldn't even go through customs until you arrive in Naples or Rome.
If they are on different airlines, or more important, different airline alliances then you are much more likely to have to claim your bags in Lisbon then re-check onwards to Italy.

Single ticket you might make it. You go through immigration in Lisbon and customs in Rome. In the immigration area look for a short connection line. I would try with one ticket but not two independent tickets. If two tickets, you will go through customs -- not more than walking through the green door - but now you are outside of security so that is another line to get back in and on to the next gate.