Depends on what you mean by "important."
Most likely, you're referring to how medical schools view your final grade in the course. Calculus is not required by all medical schools, but many do look for it (or at least a semester of something relatively equivalent) or highly recommend taking it to be more competitive. Therefore, it is in your best interests to do as well as possible. I'd also encourage you to take Calculus II down the line, to maximize your potential options when applying. If you're worried that your professor is going to be a harsh grader and want to optimize your chances of receiving a good grade, then why not take it with someone else? Medical schools are unlikely to be sympathetic of any plight involving a professor who is too difficult (they can't be, really, given how many other factors they need to consider).
If you're referring to calculus' relevance to medical school curricula/the medical profession, then do not worry too much. With the exception of certain specialties, I get the sense from having spoken to various physicians that you do not need to go beyond basic algebra/statistics to be successful.
