Leaderboard may be the wrong term. In StarCraft 2 for instance, there are Bronze, Silver, Gold, Platinum, Diamond, Masters, and Grandmasters leagues. Diamond 1 to GM might include around 5% of the ranked players.
Ranked players maybe top 10% of all players? Purely guessing.
No, although the median of the bell curve of ranked players is probably further to the right than the median of the bell curve of unranked players.
In SC2, unranked and ranked players are actually mixed in matchmaking. So if you are playing unranked you still have an MMR, just not one you can see. As an unranked player, you can tell what division you are in based on the portrait frame of your (ranked) opponents.