Thickness is what matters. If you can get a rough textured surface it may also help. I have a room with cork floors and it is actually quite bright sounding not much better than regular hard wood flooring (which is not good) - so a lot will depend on the type of cork/thickness - every doubling in thickness will extend the LF broadband absorption. At 3/16" you are likely to only effect upper frequencies and will miss most of the midrange although if you add this to the gyprock wall thickness it may get you some added absorption down to 3000 Hz. For comparison think of how thick a wall to wall carpet and rubber underlay is and how it covers a larger surface of the room - that is what gives significant impact in a room.