Well, legal writing *can* be an indicator of how you do overall, but it might not be. I think for some people in my class, legal writing was their lowest grade, and they did much better in their other classes.
The only reason I think it has potential to be an indicator is that I feel like a lot of what you do in legal writing is applicable to essay exams. You're writing about the law, formulating an argument, etc. etc. But I guess it would depend on why your legal writing professor gave you a C. But look on the bright side: writing memos and writing exams are not exactly the same, you have a few weeks to read through your memo, see where you could do better, and incorporate that advice while practicing for finals.