Opencritic currently has it at 82/100. http://opencritic.com/game/4214/splatoon-2 People should use Opencritic over Metacritic because they are more transparent, and you can blacklist any reviewer you don't like from the aggregate score.
IMO Splatoon released at 81 and slowly went to a 9.5 from all the updates. Looks like they are following the same formula with the sequel, so 82/100 is a good score.
Edit: Nintendolife gave it a 10/10 but that isn't showing up on Opencritic yet. This should balance out Slant Magazine's terrible score of 6/10 (They gave BotW 6/10 too).
Then every user will have a different score.
They give the game two scores. One is your own personalized score, and the other is the official score. The official score is kind of hidden, while the personalized score is put on the front page of the site. I think this is a good thing though, because you can get a score based on reviewers that you like or respect. So at a glance you can see what people who like the same things as you thought about th game. I always check the score and then read reviews from sites I like to get a better idea of what the game is like.