There is a good chance your robots.txt file is actively blocking the wp-content/themes folder. This is done by many SEO plugins by default. The result would be that Google can't see your CSS and JS files and will perceive your content as if it was unstyled. To fix this you have to edit the robots.txt file. You can check your robots.txt file through Google Webmaster Tools and even check if your current theme stylesheet is being blocked. If it is being blocked, simply remove the reference in robots.txt, rerun the new mobile check, and Google will flag your site as mobile friendly.