You can filter these out actually. When you go on someone's profile, and then to the "repositories" tab, there is a drop-down menu that reads "Type: All" from which you can select other repository-type filters, namely forks, mirrors, and sources (the user's own repositories).
It's somewhat imperfect as not all forks or mirrors are marked as such by github (if, say, it's a fork of a repository not originally hosted on github), but it works in the vast majority of cases.
It's somewhat imperfect as not all forks or mirrors are marked as such by github (if, say, it's a fork of a repository not originally hosted on github), but it works in the vast majority of cases.