Was Cartman's saving of Kyle in imaginationland and smug-alert selfish?

Yes, Cartman's motivations were inherently selfish. Cartman revived Kyle in Imaginationland in order to continue his humiliation rather than a true concern for his wellbeing. It's important to note that Cartman only gets furious and attempts to revive him (in a direct parody of the revival scene from "The Abyss" no less) after being reminded by the general that Kyle could no longer suck his balls. Up until that point, Cartman looks more confused than anything. This is further backed by the extended version where Cartman exclaims, "He can still suck my balls! Somebody get him some lemons and some chapstick right away!" directly after Kyle is saved. The episode smug alert has Cartman react to Kyle's leaving with initial joy, only to fall into boredom without Kyle's angry reactions to feed off of. Basically, despite hating Kyle, he does need him for the sole purpose of amusement. And from what Go God Go has shown us, Cartman will go through drastic lengths just to avoid boredom.

If this were any other character, their motivations would be pure. But because it's Cartman, his actions are inherently selfish. Cartman's other sociopathic traits of sadisim and a dire need for stimulation are amped up to 11 in these scenarios for comedy's sake. It's exactly why Cartman is such a twisted yet morbidly interesting character to begin with.