YouTube rates country songs the same way it rates all other music on the YouTube website, with evaluations based on the Recording Industry Association of America's Parental Advisory Label Program. This program labels content based on criterion of strong language or depictions of violence, sex or substance abuse.
Due to the wide variety of possible content governed by the Parental Advisory Label Program, there is no uniform way to apply this criterion. The Recording Industry Association of America asks record labels or the artists themselves to make their own subjective decision as to whether or not they should apply the label to any given song. So, any content rating you may see of a country song on YouTube is a rating that has been given to the song by either that song's artist or the artist's record label.
YouTube itself does not apply ratings to content on its website unless requested. Evaluative ratings such as the thumbs up or down count on a video are done solely by the community, not by YouTube. Content on YouTube can be labeled as "age-restricted," which prevents the video from being viewed by anyone that is not both signed into YouTube and over the age of 18. However, this restriction can only be placed on a video by either the uploader, or by a YouTube staff member if enough community members flag the video for review and the video is determined to contain offensive content.