There is a sense in which other peoples stupidity is no business of mine. If they want to wear mouth diapers as a repository of millions of bacteria, while filtering no viruses in either direction, that is their own business.
But I think most of hs rightly fear mass stupidity as a latent threat. For one thing, the stupid sooner or later tend to close ranks and punish all dissent and dissenters.
For another, if we take the notion of community seriously, it is a linc series of reciprocities which the stupid cannot be depended on to fulfill faithfully or competently.
So anger in response to stupidity amounts to a latent sense of threat which is real enough.
The task of course is to recognize this, feel this, and get through with the knowledge now conscious, since the emotion provided the needed message.