I play almost exclusively quick play, but I did do some ranked matches just to get placed to see where I end up. As far as I can tell, my opponents in ranked were about the same skill level as the people I was facing in quick play. Since I did so few ranked matches, I was wondering if my quick play MMR (or whatever the equivalent is in Overwatch) was separate from my ranked MMR.
This became abundantly clear if you played QuickPlay in the off season between Competitive seasons 1 and 2 and again between seasons 2 and 3.
Why? Because people who played exclusively Competitive were queuing with the same QuickPlay MMR they had before they switched over to playing Competitive, meaning that they were often being matched against teams whose players were considerably worse at the game.