Mine did not appear to be installed upside down as the key hole was oriented correctly. Plus the new one I installed, that fixed the monitor sag issue for me, was installed the same way.
IMHO, the way to tell if the monitor mount was installed upside down would be to look at the center screw and the key. If the large hole in the key is at the bottom, and the slot is at the top, the monitor mount is installed correctly - not that I know that the mount being installed upside down would cause a sag.
However, if your monitor is sagging, and if that is caused from the monitor mount being bent, then removing the monitor and the mount, flipping the mount upside down and reinstalling the mount might, maybe, fix the sag problem.