Hearthstone Lead Data Scientist, Tian Ding, has released a breakdown of where you stand in the population based on your ranking in the Mercenaries PVP mode, the Fighting Pit. If you've managed to hit the top 200 leaderboards in any of the Hearthstone regions, you are solidly in the top 1%.
You can find Tian's post down below. If you want to learn more about how matchmaking works in the mode, you should check out the dedicated post that Tian made as Mercenaries was launching earlier this week.
Launch week's Mercenaries PvP external rating (at least 5 games played):
2100 – top 50%
2600 – top 40%
3500 – top 30%
4500 – top 20%
5100 – top 10%
5600 – top 5%
6400 – top 1%
We check PvP stats from time to time and may or may not tune things in the future.