100 damage is pretty strong for a legendary signature move.
Plus, don't forget the fact that it can be any type when it Arceus holds a plate (Ex: Judgement is a grass type move when used by Arceus holding a grass plate). Gamfreak didn't want to make the most powerful Pokemon have an overpowered move.
Keep in mind 100 damage is quite strong for even a signature move. And considering it can have stab to whatever type (Accompanied plates,) you have a 100 power stab to what ever type you want so with a little prepping, it can be the most effective move in the game.
Game Freak designed it that way.
The move already has the twist of changing it's type, and to be honest higher than 100 power would be too overpowered for Arceus' liking. 100 power isn't lacking anything, for the base power of a move, it is a solid choice.