Retaking an undergraduate course due to poor grade before graduate school applications
Yes, it would look (a bit) better. Yes, focusing on other things is probably (not necessarily) a better use of your time. If the course is a foundation course in what you intend to pursue then it might be more important, but mostly because you have missed some necessary insights. 

A single "bad" grade isn't likely to mean much of anything as the admission process looks at many things and isn't, fundamentally, driven by a few numbers. You might be far better off taking the next course in sequence if any and setting yourself up for still more advanced work than to "stall" for a bit while you make things up. 

It feels like "grasping at straws" to me. If this would be an essential block, then you probably have far more important issues to deal with. 

But do what you can to do better in future, possibly by asking more questions or doing more "exercises" in your courses.