mobile usability issues with /wp-content/uploads/ folders

Shortly – It matter now, and you will get penalized.
Long story:
You should give more details, at least an url where the error comes… There is huge possibility you’ll get penalized for that, as the new Google update require access to files that were ok to be blocked until now. For example – blocked .css or .js from Robots.txt that is in the wp-content directory could hurt your mobile usability. For that you get direct penalty by the last mobilegeddon update, as the resources blocked by robots.txt and are vital for the page rendering are preventing the googlebot to see that page as mobile friendly.