Luffy is the only true idiot of the crew, at least when it comes to non emotional matters.
Usopp, Brook, Zoro, Sanji and even Franky (let's not forget about the Baltimore lab lol) are only idiots sparingly when it comes to gags and lighthearted moments.
Zoro i'd say is more on the simple and ignorant side of things. He's clearly not studied in a field like you mentioned, but also seems oblivious(or just doesn't care) to a lot of the world's politics and positions other than the strongest swordsman, being pretty one minded about his goal much like Luffy. But unlike Luffy, he doesn't lack common sense and is more logical of who he chooses to trust.
Still, the lack of sense of direction, the fact that he took the longest to get Luffy's message, and his general unawareness and disinterest about a lot of the world around him show that he is still probably the second least informed Straw Hat after Luffy, and there's nothing wrong with that.
The "idiot trio" line is very much also just a gag and means nothing. We know Usopp acts like a clown but has at least some knowledge about gadgets and repairs, and Sanji might be a fool around women but has also pulled off some pretty clever plans on his time with the Straw Hats.