How much dumb a person has to be in order to call Zoro dumb.
As you said, Sanji is dumb when it comes to women. Zoro is dumb when it comes to direction. Only that.
Zoro even after Franky, Brook, Jinbei is the most rational person at the crew. Not saying about being smart/intelligent but don't letting him go because of emotions. Zoro most of time is the least emotional strawhat, he knows the weight of his position in crew that is even heavier than captain's since the captain in a morrow that needs the vice-captain to put him in order.
Zoro would never be captain, he doesn't have the social skills Luffy has to be one. But in a similar way, Luffy would never be vice-capitain or the crew as a whole would never survived until now without Zoro, and this is not even about his strength. But for sure without Zoro Strawhats are losing at Alabasta and Dressrosa.