The main antagonist is just the character who is the cause of the main conflict of the story. Though they often are a personal enemy, that's not a requirement.

Evil Morty is the main antagonist of Rick and Morty and while yes he has confronted the protagonists, thier relationship is not very direct like with Ricks archenemy Rick Prime. Overall, Evil Morty's goal is just to get away from the Central Finite Curve and is more antagonistic towards the citadel as a whole than anything (some of which are bad people like the Shadow Council of Ricks.) Heck, in the latest episode he even teams up with the heroes.

If we ignore the “archenemy” part together we can say The Core is the main antagonist of Amphibia and King Andrias is the secondary antagonist of Amphibia.

Even though Snoke is not Rey’s archenemy it’s just I think he’s the main antagonist of the Star Wars trilogy because he’s known to be the main antagonist of two movies (Star Wars: The Force Awakens and Star Wars: The Last Jedi), him making Kylo Ren evil is more direct than Emperor Palpatine making Kylo Ren evil and him being the reason why Kylo Ren and Rey are archenemies is more direct than Emperor Palpatine being the reason why Kylo Ren and Rey are archenemies.