Not necessarily. If you're only staying in Lisbon, you can manage perfectly well without a car. The metro, buses, and taxis connect the airport with the center, and within the city you can reach most places on foot or by public transport. That said, if you want to visit Sintra, Cascais, the coast, or take road trips into the Alentejo, having a rental car is highly recommended. It gives you the flexibility to combine several destinations in one day and travel entirely at your own pace.