This has probably been Marner's best game of the tournament so far, so I don't see how he's been bad. He's done a much better job of making safer plays and retaining the puck than in previous games. The problem is all that possession has been on the perimeter, and that isn't just on him. Hopefully he builds on this though and in his next game continues to make safe plays while also getting to the inside more.
Edit: Like he just did and scored a goal.