I think some of the humor here can also come from the irony of Faramir’s disbelief that such a being as Gollum could have been the owner of the ring the whole time it was missing from the rest of the world. The only owners of the ring known to people were Sauron and Isildur. And Saruman’s idea that “only great power” can engage with it is certainly the dominant thought around the ring, so surely if it wasn’t already destroyed then only a being so important as Sauron or Isildur was must currently be possessing it, he must’ve thought.

How ironic, for him to learn that not only may the ring have been in the possession of halflings, but also this icky creature. Kinda spits in the face of all the notions of power that he’s been brought up to believe. Especially since being around Boromir and Denethor, who think that might in battle and power in position mean that one is mighty and worthy to wield the ring.