There are of course moon landing deniers who legit believe that. Where would you put them on the spectrum of stupid belief systems when compared to say flat earthers or sovereign citizens?

Moon landing denial predates the internet, whereas the others I don’t think so. (Unless in the case of flat earth you go back far enough and count the illiterate and uneducated masses.)

You can also add JFK and 9/11 conspiracies to that spectrum. Though I’d say those are much more “mainstream” as conspiracy theories go.