I like to think that if Zim and Dib ever became permanent allies or friends, Zim would start showing tokens of Irken goodwill. AKA acting like a cat and dragging dead things to Dib.
Like, someone at skool made fun of Dib’s shirt. So the next day, Zim beats the guy up and drags him to Dib’s door. “The mighty Zim has vanquished your enemy! Show me your gratitude, Dib thing!”
Or Zim kills a bumblebee. And comes pounding Dib’s door at 3 AM so he can gift him with the bee corpse. “Accept this amazing token of my generosity!”
And Dib would just be so done with him. So done.