It's at the top of my GCSE results and it's not a very good grade... I got a C because my teacher told me it wouldn't really be seen anywhere because it's not a GCSE so I didn't take it seriously and didn't revise (big mistake, I know).
I am aiming to go to a top university like Oxford, Cambridge or Harvard but I'm scared that when they see that C in FSMQ Add Maths and just reject me
So my question is: does this qualification matter to universities or am I just overthinking it? And if not, will they find out about it?
That's a really good grade, so you have nothing to worry about. The only reason some maths teachers teach their classes to do further maths is if the class is already doing very well in normal maths and want to teach something extra and so gain a gcse level qualification at the end of it.
They won't base their decision on that one qualification alone, don't let this concern you and focus on smashing A levels