The concept of ready check is good, the implementation need some polishing.
As stated by Zirael, need 3 answer : Ready, not ready, "didn't answer/afk" And it shouldn't pop in front of everything for everyone, especialy when engage in combat.

This is true for every popup like Friend request or even DF when a partie as been found.
For example, when you're crafting and it pops up making you fail something when you couldn't even accept the message... Just make the popup on a corner of the screen, without focus on it, just to say "hey, it's ready when you are" and not "hey, clic me now, it won't work !" making you loose more time than it need to finish you're job and accept request.