The most important thing ...
The most important thing is understanding the material. Grades are secondary. How you learn when you're young is typically how you'll approach the rest of your life. It's best to learn best practices when you're young. Early education is foundational and theory. It's designed so you can learn knowledge and absorb experiences better when you're older.
Of course if you're getting C and below in more than a class or two, it's likely you're not understanding the material. If you cannot understand the basic academics and, more importantly, common applications, then you're likely to be just another one of the masses that get taken advantage of regularly.
Most engineers are not high IQ students (typically only 105-120) and they have to do all of their homework to understand it completely. They are methodical and stress not getting an answer, but understanding how to arrive at the answer. It starts with them understanding why algebra is important to learn (whereas people of IQ 125-150+ think algebra is a stupid idea), and then they just keep learning tools and how to apply them. Using calculus to describe microeconomic systems and engineering mechanics becomes second nature, literally.
I was clearly an A/B average student in secondary school and a solid B average student in college. I was more worried about understanding how to get to a solution than the answer.
Sure, there were perfect GPA 4.0 engineering students who hardly had to study. They'd look at a problem and know how to derive a system of equations without many exercises. But they were very, very rare. Most of us had to review, analyze, derive, get frustrated and do it over and over before it become more commonplace in our knowledge.