I just received a similar request for the website a few days ago. This was my answer:
I'm afraid this won't be possible. Apart from technical difficulties, the main problem is that downloads are counted per file/version. But in the module list, you see modules. So which number should be used? Latest version? Then even a popular module which is updated often will usually have a low number. Total number for all versions? A module with many version obviously has more downloads than one with few updates (but same number of active users). This only works well in Play Store because you have a Google account and they keep track which apps you have installed. I don't know anything about that for Xposed users.
I should also add that I don't want to add user accounts as it comes with a lot of responsibility and effort.
In the app itself, the download counter isn't available at all because it changes rapidly, which isn't reconcilable with the heavy caching that is taking place in various places in order to keep the server load and traffic reasonable.