What is this trophy room for?
Solution 1:
After doing some research, it appears that this is a reference to the original NIER, as the entire library is a replica of Devola and Popola's library from that game. From this thread:
The easter egg is that the whole library is from the first NieR. In the exact same room in the first NieR there were trophies as well but they were trophies of the bosses in that game.
This trophy room is another feature of the original NIER, in which trophies representing all of the defeated bosses were displayed. This room is identical, except the bosses displayed are from NieR: Automata, rather than the original NIER. An image of the original NIER's trophy room can be seen on the wiki page:
