I have been following posts on Facebook on the various Lisbon/ Portugal pages and there are many recommendations for Tuk Tuks on those pages. These are not the kind that sit around and wait for a passenger (like a taxi). But, rather, they are licensed guides that give you a tour of the city - some are private and some are maximum of 6 passengers (tuk tuks are small).
So, if you are wondering about an actual city tour of sorts with a Tuk Tuk - make sure they have good reviews and are licensed . The ones I see on FB have all that. You can also go on Trip Advisor and search.... I would not book via Viator or Get your Guide - simply because they are not tour operators - and they take commission from the actual tour company. If you click on a tour, find the word 'operator' - and then go to their website. I wouldn't just 'grab/hail" a tuk tuk off the street though.
We will be taking a Tuk Tuk tour on our trip - but I still need to finalize other details.
I have read that Bolt and Uber work well also. (I don't use Uber in the US , but may have to use Bolt over there!)