To be honest, this strip confused me, too, when I was a kid. I remember calling one of my brothers “homely” after I read it because I thought it was a compliment. Boy, did my mother ever set me straight about what the word REALLY means!
No, you’re not missing anything. The joke is that Hobbes can’t really do math, so he put down “Atlanta, Georgia” as though the question had to do with Geography or History as opposed to 1st-grade Arithmetic.
Obviously, or else you would have gotten the joke!