"Tehát nem én vagyok hülye, hanem te!"
Translation:So I am not stupid, but you are!
It sounds incomplete without the verb at the end. "Therefore I'm not stupid, but you are" would be good.
Actually I think it sounds even better (and matches the meaning better) if you leave out the conjunction, and say: "Therefore I'm not stupid, you are."
The word order "... not I am ..." in the suggested translation sounds awkward in English. Not wrong, but not normal.