We just came back from a week-long trip to Lisbon and it was surprisingly more crowded than I had anticipated, given that it's a low season. I suspect the summer months might be worse if you go to popular places like Sintra, for example. Or even within Lisbon, popular attractions like the Jeronimos Monastry might have a long line.
There are a few restaurants that specifically serve seafood only, but the majority do have other meat options in their menu. My daughter is allergic to seafood and we didn't find it a problem when we ordered seafood while she chose other meat or vegetarian options in most restaurants. One thing I would highly recommend is to book restaurants using the app TheFork. Throughout our week-long stay, we used the app to book half of the restaurants where we planned to eat and got 30% discount off the food in most of them! Saved us quite a lot just on meals alone for the four of us. A few of the excellent restaurants where we ate using the App with 30% discount come to mind--Sebastiao, Alpendre and Animal Restaurant & Bar.