WordPress 4.4 introduces a new feature called Responsive Images, this enables WordPress to serve smaller images on mobile devices which saves bandwidth and also improves loading time. Builder supports this feature as well and images you add in Text or Image module are automatically made responsive.
On mobile, "Thumbnail" image size is used, it's 150 * 150 by default which could be small for mobile display and result in images of low resolution. To fix this you can go to Settings > Media from the admin menu, adjust the Thumbnail image size (set to ~ 400 pixels for example) and then regenerate the thumbnails (see: https://wordpress.org/plugins/regenerate-thumbnails/) to apply the changes. Now you should see better image qualities on mobile.