Has the Twelve Demon Moons power levels been given by the author?
Their rankings are denoted by their numbers. Upper Moon 1 through 6 are the strongest 6 after Muzan arranged in the order of their numbers, then lower 6 stand in a similar fashion. 

Here's some info from the manga:


   Upper moon 1 was annoyed that Upper moon 3 Akaza did not abide to the hierarchy of Upper moons. He suggested that if this bothers him so much, he can apply for a Blood battle against Upper Moon 2 to swap places implying the hierarchy is based on battle strength


This is what we can assume given the latest chapters:


  There are some demons with the potential to be one of the moons. As we see that Upper moon 6 (Daki and Gyutaro) and Upper moon 4 (Hantengu) were replaced in a matter of days following their deaths.