It's not just about being smart/dumb, it's also about being trusting and agreeable. I think falling prey to the occasional (hopefully not life-destroying) deception is part of the cost of being an easy to work with and trusting person. There are people who will take advantage of you, but most won't. On the balance, this leads to better relationships and a better life. OTOH you can be cynical and questioning of every persons' motives, and that can make many normal interactions exhausting, at work and in social contexts. It can add an edge to you that most people do not like.
The ideal would be to have perfect, instant judgement of character, knowing when to be open or closed with people in various contexts. That's the hardest of all, and comes from having great parents/mentors and making lots of mistakes.