Does mIRC automatically cascade the windows in order of most talkative to least talkative?

It's just you. The windows are tiled/cascaded (depending on which you choose) in order from the current active window to the last active.
eg: If you have 3 windows, with the 2nd one being active, but the one you used before it was window3, and window1 before it, then the order will be Window2, Window3, Window1. It's based on your activity in the window, not the general activity of the window.