score:3
I was just in Lisbon and can confirm that this is doable. The metro takes 45 minutes to an hour to get to the city center. That would still leave you plenty of time to explore parts of the famous Bairro Alto or Alfama areas. There are also buses and Uber is an option as well. Given that there's not much to do at the Lisbon airport, with that much time I would certainly head to town for a bit. Though do leave yourself plenty of time to get back to the airport. European airports in general are in a bit of a crunch this summer and you'd hate to miss your connection.
Upvote:2
Yes, you have more than enough time to go to the center, where there is a lot to see. You could possibly also go to the BelΓ©m area, where there are some major sights but that would add some time and hassle if going by public transportation.