Qanon was purely a creation of the internet. Now go take a look at how many people believe one, many or all of the various Qanon alternative facts.
They don't prove that it makes people dumber. You have to quantify and qualify people and to define "dumb".
Maybe people are not actually dumber because of the internet, but the internet is very good at spreading ideas, including incredibly dumb ones, especially (because of how human beings work) those likely to cause feeling of outrage.
Maybe people are not dumber, just too defenseless against the scale of bullshit they are faced with because of the internet. Maybe internet is an incredibly good tool, but strongly requires good learning / training of critical thinking and there's not enough of this yet.
It's hard to argue that nowadays people believe more crazy stuff than before the internet was invented. (It's very easy, of course, to claim this, as many like to do.)