I think he's having a very good year. He is a consummate pro. He works hard every game and every shift.I'm not saying he's a bad player.
He's a point a game player and that alone makes him good.
In a salary cap era though players are evaluated based on their cap hit.
Compare Matthews and Tavares and Matthews is way better.
Compare Marner and Tavares and Marner is way better.
Compare Nylander and Tavares and Nylander is way better.
But you asked why his +/- was only +1 (it is now even after last night's game) and the easy answer is he is not a good defender and doesn't generate enough offense even strength to compensate.
He has 13 PP goals tied with Matthews for the lead and 18 PP assists 1 more than Nylander.
Hence PP is where he's most effective and PP doesn't count for +/-
He does the dirty work in the corners and in front of the net. What metric measures battles in front of the net to get in position for tips and screens? What metric measures winning battles in the corners? He's got a bit of nastiness as well. Matthews, Marner and especially Nylander have learned a lot from him as far as working hard at both ends of the ice. What metric measures that? He's the least flashy but he is clearly the captain. The other players respect him greatly.
There's a lot of intangibles that he brings.