Saturn is more than adequately written if we're being frank. There are two major things he's meant to do as an antagonist. Firstly, through the Kuma flashback the story successfully establishes the sheer degree of his vileness – the thing that makes things like Kuma smacking him in the schnoz or Imu skeletonizing him so cathartic and satisfying. Secondly, he's meant to show that celestial dragons, even high-ranking ones, are indeed moronic dipshits. In Saturn's case it's not only about his repeated incompetence (I do think Oda was intentionally playing up just how much he was getting bodied both physically and intellectually throughout the arc), it's also about his utter hypocrisy. Despite his whole schtick of lamenting the fact that "oh, why must you insects disobey orders... if only you did what you were told to," he was the one who kept Emeth around instead of sending him to a scrapyard (supposedly an order from Imu), all because of his own interest and curiosity. That's a somewhat compelling point that many seem to kinda miss imo.
What makes him look kinda crap is that all the other Gorosei seemed to be doing their best impression of "muh iNsEcTz" once they entered Egghead, but that's not really a problem with Sadturd's own characterization.