I can switch to 5v5 goal metrics and shot metrics if you prefer, but the point isn't to pinpoint just how bad he has been, I'm not looking for precision. There are certainly many factors that go into a terrible plus minus and some of them (like being on a bad team with suspect goaltending) are at play here, but you'd have to be blind or willfully ignorant not to see how bad Karlsson has been defensively this year, and he hasn't come close to making up for it with his offense since his first 11 games. That's the point. He isn't helping matters right now, like at all.
He gets walked on a daily basis. He loses his man when covering the crease, he's constantly shooting pucks off guys shin pads creating breaks in the other direction, he's no longer automatically first on the puck, and hasn't got the first step to separate himself and create lanes when he does get it first. He's seemingly lost confidence in his pivots to the point that he's back to turning the wrong way like he did in 13-14. He used to create offense out of nothing, but that isn't really happening anymore.
His poor plus minus is a symptom of his poor play; it's not a perfect reflection of how bad he's been, and he's not the sole reason it's so bad, but he's been a major contributor to it without a doubt. I'm not advocating +/- as means of comparing his play across the league, just using it to illustrate that goals are going in the net at a ridiculous rate right now when he's out there. 2nd worst GA/60 rel among d with 500 mins in the league. 1st in the league in GA/60 among that same group, a full goal per 60 worse than he career worst. He hasn't had a rel GA/60 this bad since his rookie and sophomore season. Next closest is 2013-14 when he was objectively bad defensively. but his rel GA are a good step better that year.
Right now, Karlsson is simply not helping this team. That's the point I'm trying to make. You're right, +/- is terrible, and pacing it out is typically not going to be a fair representation of a players play, but in this case, it's honestly not far from it.