This X being unavailable can be just incompetence. Esp on Android with all those slightly different screen sizes.

Recently i tried to run an app on an iphone mini but their layout was crap and both buttons on the first screen of the initial setup were on top of each other. Couldn't click either :)

Incompetence that for a curious coincidence advantages them and frauds both the user and the companies paying for ads.

For sure mobile screen variety isn't helping, but I don't believe that's the reason the X button doesn't work on ads.

I've seen many ads where the first "X" button that appears is fake, placed there to make you question yourself: "Well, the X is very small, it appears I fat-fingered it". It then becomes "real" after you switch out of whatever the ad opened and back to the app hosting the ad. It works on the second try, and that confirms your belief that it must have been your fault.

Well I've also seen my wife playing a F2P game and sometimes the ads either crashed the app completely or locked up with her unable to click on either the ad or close. When they worked the close button was available.

I'm not rich enough to play free to play games so I have no personal experience.