Yes, anyone can rate a blog as mature from the Reader. A rule of thumb a lot of people use is "Would I want this post showing up on my screen with my children in the room?" Not everyone, of course, but that's a bar that you can keep in mind if you so choose.
The mature rating removes the blog from the Reader. This is specifically what becomes restricted to a Mature blog:
*It is not eligible to be featured in Freshly Pressed on the front page of WordPress.com
*It will not appear in global tag listings
*It will not appear in Top Blogs listings, recent posts, or related posts listings on other blogs
*The blog’s users cannot use that URL as a link in their WordPress.com Forums profile
I tested commenting on your blog from a regular user account and I was able to comment just fine. I then trashed the comment, so if you see it in the trash, that's what happened. So none of your users should have a problem commenting. If they are having an issue commenting, they may want to clear their cache and try again.
The entire image would need to be removed to remove the mature rating. I have asked several people if they consider the image abstract and no one considers it to be anything other than extremely realistic.