Do you to be extremely smart to become an engineer?

An engineer is someone who enjoys solving problems and is persistent at finding resources to help her come up with a solution(s). Being smart is a gift and one can utilize her smarts in day-to-day living.  One can become smarter through learning from one's mistakes.  I don't think one has to be smart to become an engineer.  One has to enjoy her profession whatever it is and it doesn't have to be engineering.  

The field of engineering is so vast and interdisciplinary. For instance, one who doesn't enjoy Math or applying Math in problem-solving but enjoys studying scientific facts and doing scientific experiments in the fields of Biology and/or Chemistry doesn't have to be smart in Math, but can still become a rock solid bio-engineer or chemical engineer.  One who enjoys tinkering with tools and building can explore Mechanical or Civil Engineering, even though she needs help in Math.  She can build her Math skills through taking enough courses to help her in later designing objects from toys to buildings.

One can be an excellent engineer and not "smart" in other aspects of her life. Enjoying what you do best is the most important ingredient.  You will find it in your passion(s).