The mirror matches in "Play" mode is getting silly. As soon as I step out of ranked play and want to chill in "play" I get mirror'd. Boring.

Yeah, I think I've been experiencing that too - if I go into BO1 unranked mode with RDW, chances are I play against another RDW deck, but once I switch to another deck all the RDW decks disappear. I wonder why.

(This info might be outdated)
It is because they are trying to match you based on deck strength. I think they measure the strength of each card based on how many wildcards have been spent crafting it and some other metrics. Then they total up the power of all of the cards in your deck and try to find you another deck with a similar total. It seems like a good idea but it is too precise. Instead of an opponent with about the same number of powerful cards, it often finds an opponent with several of the exact same cards.
It also caused 100+ card decks to get matched up against strong opponents because they had a high total of card power.