Maybe, but there was something very Nietzschean about Maxima when she was first introduced. She is certainly repressing a lot of things when she breaks with the tradition of her people by refusing to choose other warlord suitors picked out for her. I'm just pointing out that Maxima may simply recognize instinctively that mercy is the privilege of the most powerful man when she watches Superman disobey Mongul and refuse to kill Draaga. So, that might explain why someone who is at the apex of an intergalactic empire might not be interested in someone like Zod. Superman on the other hand has a lot of power, and he's not really trying so hard per se.
Take her appearance in JLQ 13 about 5 years after her introduction; Maxima stopped Ultraa from killing Captain Atom in her name, saying that a warrior doesn't kill a downed opponent. Wounded, Ultraa argued that a warrior wins, but Maxima explained that Earth has taught her so much about unselfishness and mercy. Again, I think you could take her words at face value, or perhaps she's still inventing explanations that hide or deny her true motivations. She's complicated.