This is why it says it will "prefer" not to make mirror matches. Because of the huge number of people queuing as launch day heroes, those are still going to see mirrors until demand drops down far enough that it's no longer necessary to mirror in order to create matches in a reasonable time. Normal day-to-day heroes, though, shouldn't see mirrors except in exceptional cases.

Playing a selected under-represented hero makes the matchmaking slightly easier, but except on release day, doesn't do a whole lot to help the overall system. Hero collisions have become less and less of a problem as we've expanded the hero pool (which is why we can make this mirroring change now); the meta configuration of your selected hero is more impactful.

An example: Chromie is a less popular hero in Quick Match, so you might expect that playing Chromie instead of Li Ming would help the system out, Li Ming being super popular. However, Chromie and Li Ming have the same hero meta, so they're essentially interchangeable for that part of the matchmaking process.

Auto Select, on the other hand, lets us use you as a wild card in any match with players close to your rating. It's especially helpful if there's at least one healer, tank, bruiser, and miscellaneous damage in your auto select filter; we can stick you literally anywhere to get the teams to match up.

way into being mirrored, due to more people wanting to play those either to try stuff they don't have or

I'm just picking my words carefully so folks don't come back later and say "You said only launch day heroes!" ;)

It's not quite 90%, but he is over-represented. It's because he's the only hero who's both Ranged and a Tank. Since we cap the number of melee heroes the matchmaker will put together on a team, and we require that if one team has a Tank, the other team must have one as well, Rexxar slides into a lot more compositions than other Tanks or Ranged heroes.