No, unrequired bosses are fine. Many of the post-Plantera bosses or events, or Solar Eclipse and Queen Slime are not required because they can be hard, but provide the player with good loot. Newer players can skip them if they wish.
Also, If you make every boss and event required, then progression becomes repetitive and bland and the game loses replayability. Optional bosses give the player choice and variety in progression.
Anyway, if you use lunar pillar weapons to kill Empress, then you’ve passed the utility for her drops anyway and it doesn’t do much for you.