The answer to your question is so obvious that I suspect I do not understand it.
My assessment:
- It certainly is not a subset since most of the language has NWN special functions.
- Is it a superset? No. Is it like one? More or less I suppose. It can't do everything that C does as far as I am aware so this is not really true, but its C-ish syntax more or less behaves as C does.
More importantly, I don't think the answer to that question really matters. NWScript is not C, but behaves like C. So if you know C or any C-like languages you will be right at home very soon. I am not a developer and have no education in coding, but I figured it out fairly quickly. You should do fine.