Mobile looks a lot better for us when we track call conversions.
We can't fully close the loop. We only know the average value of a call and how many calls come from Google Ads either directly (clicked on a number in the ad) or indirectly (clicked on a link in the ad and a number on the web page).
If you have an e-comm biz without a phone sales component, I would guess that mobile conversion would be atrocious. Though we do see some evidence that people have a "research on mobile, buy on desktop" pattern. It's a very loose correlation though. For example, we'll see a spike in mobile traffic followed by a jump in desktop conversion rate.