I'm not sure that is mostly the ad's fault. Hitting a target on a touchscreen is hard to do. This seems like it's the phone's fault first to me.
(If you're using a mouse, forget what I said. But I haven't run into an ad where the close button didn't close it... if you were able to click the close button.)
(If you're using a mouse, forget what I said. But I haven't run into an ad where the close button didn't close it... if you were able to click the close button.)