Hmm seems that even forum mods agree.
Mand -
"Your frustration is entirely real and valid, and why I strongly believe MMR should be class-based, not account-based. What you are experiencing is the kind of transient MMR failure I described above - when you swith to a weaker deck, you no longer have fair matches. And after losing a bunch and switching back, you have MMR failure again, as you proceed to stomp upwards again.
Ideally, each deck would have its own rating, but practically that can't work. Class-based MMR is a reasonable compromise between having enough games to make MMR work at all, minimizing info that needs to be communicated to the player, and ensuring fair games as much as possible."
So if I have understood the match making right - your games are set at 50% win/loss based on your skills & level.
Crap players with rare/higher cards or players who deliberately lower their ranked MMR (to get gold for daily quests) to stomp newbies in casual get paired together.
I hope they look at the MMR then and try to make it fairer, perhaps as suggested on their forums - tiered matches or class based for that class level would make more sense if it can be done.
Failing that I guess I will just have to L2P