Grimoire isn't related to skill, but it can be used as a good way to get an idea of how good a player is. More often than not, the players with higher Grimoire are better because to get that high score takes dedication to the game and shows that they have, in all likelihood, spent more time and have completed more endgame activities. Of course, there will also be people that have an interest in the backstory and those who want a high score for the sake of having a high score, both of these types of players could be entirely awful; you don't know until you play with them.