Nostr is not ad-free. At the protocol level, yes we have no NIP for ads, but at the relay, client and user level we are not protected from ads.
To tell you the truth, because of spam, I see ads in Nostr even more often than anywhere else.
Discussion
You see ads maybe because there are not so many users as on other social media platforms? Maybe when we'll have lot of content here we'll see less ads? I need to check I am not sure I saw ads on my feeds.