zoro's saying he's not stupid to the point that he can say something absurd like he can defeat mihawk. mihawk says he doesn't understand how zoro can do this, while still acknowledging mihawk as his enemy, to put his head to the ground and ask mihawk to train him. zoro grandly responds, to defeat you, or literally, to surpass you. mihawk finds this funny, laughs, haha you're asking me to raise a student so that he can defeat me? that's pretty dumb. looks like you found something that's more important than your goals. (basically their duel-esque promise).
One more thing. Oda will take a month long break from OP after this chapt =(.