You assume engineering students are smart just because they study engineering. I'm sorry, but an engineering student who gets bad grades is just as dumb as anyone else. Some of the dumbest people I knew when doing my undergrad/Bachelors degree were engineers with bad grades.
What is the purpose of university? One of them is learning
how to learn. When you have trouble with a particular subject, you probably just keep staring at the same textbook, re-reading the same pages for a week in hopes that you'll understand it soon. What most dumb engineers, or students in general, don't think to do is use their resources. If you don't get this concept by your 4th year at uni, forget it. Regardless of your grades, you totally missed the point. If you don't understand one book, use another textbook. There's always more than one source of information, and one may be clearer than another. That's the difference between a Masters/PhD student, and an undergraduate student. On the other hand, I also believe that's the difference between a good student and a bad student. The bad student gets bad grades, but thinks he's smart.
And to top it all off, many engineers can't think in any way other than logically, making them "bad at life". That has nothing to do with being a bad student, but it does mean that many engineers are better at things like logic, but completely fail at other things such as feelings and empathy.