The writers seem to enjoy making her look dumb. I hate to use Chloe as a point of comparison, particularly since I know it's a sore subject, but Chloe admitted to having her suspicions about Clark before she caught him playing beach ball auto. Lana knows a lot more about Clark than Chloe did before Pariah. Lana's also witnessed bona fide aliens, been assaulted by them more than once, and has extremely specific information about where they're from, what harms them, and what they apparently have been trying to do. She knows Clark knew about Lex being possessed and that Clark might have to kill him, and she acquired a weapon to achieve that very task from Clark's MOTHER (huh?!) The absurdities involved with Lana NOT putting the pieces together at this point rival those of future ace reporter Lois Lane, who doesn't instantly recognize Clark Kent as Superman because he wears a pair of glasses.