Google's new Landing Pages Tool for mobile shows for each of your landing pages in search campaigns a "Mobile-friendly click rate" (MFCR) (% of clicks from smartphones that land on a mobile-friendly page) and number of clicks.
"On this new page, you’ll see which URLs in your account are mobile-friendly, which ones drive the most sales, and which ones may require your attention. For example, find the pages that get a lot of clicks, but aren't mobile-friendly. Then prioritize them for your webmaster so you can convert more of your ad clicks into sales."
Google suggests sorting landing pages by ascending MFCR to prioritize pages to be fixed, then click on a landing page to open the Mobile-Friendly Test to find out what you need to do.
AdWords success is ever a moving target.