No, the mirror isn't prophetic as such. But like you've said, if you want something, you will work for it, so it can become true. And that was somewhat true for both Harry and Ron. Ron maybe didn't become Head Boy or Quidditch captain, but he eventually became Prefect and Quidditch player, which is almost as good, because he worked for it (at least for the Quidditch thing). Harry wanted to know his parents, and while he couldn't bring them back from the dead, he found out more and more about them over the years, and he even got to talk to them twice, even if they weren't more than ghosts. And a lot of that happened because he wanted to find out more about them, and because he used the Resurrection stone that once.