The NSA is heavily Mormon? Where did you get that information? I've been looking around online, and the only thing I'm coming across is conspiracy theory websites about the new Utah compound they're building.
Mormonism is the only major (ish) religion that's homegrown and culturally they're mostly WASPs or close to it. Above average chance of getting clearance and of wanting a career in a TLA. The NSA is not special in this way. The DEA, FBI etc. will show the same pattern. Difficult to prove seeing as I doubt US agencies keep records on employees' religion though.