In my humble opinion mobile ads just doesn' work for App developers as a mean to generate revenue from their apps.
You have an average CTR of 0.21% , but you only get one cent or two cent per click. Meaning that you need millions of viewers per month to get a decent revenue.
In my own experience I had an app with AdMob ads, and indeed I got a daily CTR of 0.17/0.2/0.22; much in line with the average CTR found.
But I only had a few thousand visits per day, and that was strange because my app was downloaded much more.
And than I came to conclusion that the only thing that prevented from ads to be shown, was AD BLOCKERS !
And that my friends is why the mobile ads do not work ! More and more people install ad blockers on their devices and less and less ads are shown.