Okay, not everyone. Yet
From the latest PTR Build:
* Permafrost now also reduces the target's healing received by 7/13/20%.
* Improved Mind Blast has an additional effect - while in Shadowform your Mind Blast also has a 20/40/60/80/100% chance to reduce all healing done to the target by 20% for 10 sec.
It's unlikely that these are the only two classes to get this talented Mortal Strike debuff, there's no real reason for them to get it and no-one else. A secondary effect for CS perhaps?
This strikes me as tacit acknowledgement that healing is going to be pretty over the top in 3.2 PvP, and so every offensive class will need the option of speccing for at least minor Mortal Strike in the Arena. Warriors, Rogues and Hunters obviously won't need such a talent and will continue to have an advantage at -50% rather than -20, but that won't stop the QQ.