Grimoire only gives you an idea of how much time someone has put into the game. A majority of grimoire is done through simple farming (collecting ghosts, getting kills with classes, weapon kills). Others can require a little skill, namely the 3v3 and rumble wins. But even with the 3v3 (including trials) you can be carried for all the wins. So you cannot determine skill from those. So again, grimoire is only how much time someone has put into the game. Nothing more, nothing less.