You should use Comment instead of UserComments.
UserComments is for the action/event of commenting, while
Comment represents the published comment itself. (You’ll often find that
UserComments is used although it’s not appropriate; that is because
Comment was introduced later.)
Neither Schema.org nor Microdata define which conclusions can be drawn when a page contains these annotations, i.e., it is not defined that only the main content should be annotated.
If a consumer (like a search engine) draws a conclusion like "If I find
Comment markup, I assume that these comments are about the main content of the page", it’s quite adventurous: it would probably be wrong with this conclusion more often than not.
Instead, such a consumer should look for a property relating this
Comment to something. For example, when it finds
WebPage → comment → Comment
it can conclude that these are comments about the webpage.
When there is no property, the only thing that can be concluded is: There is a comment.
So, it’s perfectly fine (and it can also be useful) to mark up any content that is shown on the page, not only the main content.