His intelligence isn't because of superpowers or radioactive waste or anything. Just simply working hard at school and getting the best education due to his wealthy father, Lionel Luthor (who's quite smart himself)
If he can build a Lexorian Warsuit with missiles, machine guns, kryptonite rays and self-propelled flying gear, then that counts as a 10/10 score on the Smart-o-meter (this is a made up device).