'Blatherer' actually seems pretty good to me too. Something similar like 'babbler,' 'gabbler,' 'jabberer' might also work. (Onomatopoeic, much?)
'Windbag,' 'motormouth,' etc. put more emphasis on the quantity, running on and on, rather than the quality of speaking nonsense.
'Person who spouts (a lot of) nonsense' might actually be the clearest, even though it's not a single word.
If the context is clearly related to anti-vaxxers or other conspiracy theorists, though, it might be necessary to use one of those terms instead, just for clarity. Or yes, 'pandemic
nutcase' is a possibility, as is 'covidiot.' /-:
I would not recommend 'blatherskite,' because it's surely regional or archaic or both. M-W, strangely, lists it as a synonym for a person, but when you click through to the definition, it's defined as the nonsense itself, not the person who spouts it.
* gossip, gossiper, talebearer, tattler, tattletale
* converser, discourser
* babbler, blabber, blabbermouth, blowhard, cackler, chatterbox, chatterer, conversationalist, gabbler, gasbag, jabberer, jay, magpie, motormouth, prattler, talker, windbaghttps://www.merriam-webster.com/thesaurus/bla...