Well, Clark DOES project a sort of blundering image when he's out of costume - intentionally missing trains, tripping over chair legs, that sort of thing. Perhaps he's a good enough actor that he's actually got Lex fooled - especially since it's been shown that even when it's staring him right in the face, he refuses to even consider the possibility that Clark Kent is Superman. Indeed, just the notion makes him furious, as it upsets his rather Darwinian view of the world ("Such power is to be constantly exploited... Such power is to be USED!"). It's quite possible that, on some level, he knows that Clark is indeed the Man of Steel, but he rejects the notion out of hand, because if his arch-enemy is indeed living peacefully as a regular shmoe out of costume, instead of using his powers to set himself up as a man of wealth and influence, that would make him *gasp!* a better man than him - and this is something that Lex Luthor will not let himself think. So he buys into the notion of Clark as a hapless bumbler instead.