Fox dominates the ratings because they have the highest-rated cable news* programs. That still only means they have a simple plurality among primetime cable TV viewers, which says zilch about the intelligence of the average American. Somewhere between 0.6% and 1% of the American population watches Fox News.
Fox viewers trust their network more than the viewers of other networks. Which still doesn't say anything about the intelligence of the average American, but it does say something about the intelligence of 1% of them.
* For legal reasons Fox News does not describe their pundit shows, e.g. Beck, Hannity, O'Reilly, as news programs. However, Nielsen rates them in the same category as news, so Fox has the top 13 shows in cable news. This is largely due to their viewers' voracious appetites for vaguely-worded assassination threats and racist jokes.
Fox viewers trust their network more than the viewers of other networks. Which still doesn't say anything about the intelligence of the average American, but it does say something about the intelligence of 1% of them.
* For legal reasons Fox News does not describe their pundit shows, e.g. Beck, Hannity, O'Reilly, as news programs. However, Nielsen rates them in the same category as news, so Fox has the top 13 shows in cable news. This is largely due to their viewers' voracious appetites for vaguely-worded assassination threats and racist jokes.