Yes, the mobile-friendliness of the website is a ranking signal, according to the official statements from Google, cited below. In April 2015, Google released a mobile-friendly ranking algorithm that was designed to give a boost to mobile-friendly pages in Google’s mobile search results.
“Today’s the day we begin globally rolling out our mobile-friendly update. We’re boosting the ranking of mobile-friendly pages on mobile search results” source
“As we mentioned in this particular change, you either have a mobile friendly page or not. It is based on the criteria we mentioned earlier, which are small font sizes, your tap targets/links to your buttons are too close together, readable content and your viewpoint. So if you have all of those and your site is mobile friendly then you benefit from the ranking change” source
“Having mobile-friendly content is still helpful for those looking at ways to perform better in mobile search results.” – source (Google)