I am not an expert, by any means. But from everything I see in Ottawa, the entire team and system seem to be tailor made to get the most and best out of Karlsson. I don`t think anything abut Ottawa is holding him back.
Just to clarify on the aspect of system as his name has come up a couple times so far in this thread:
For the entirety of Karlsson's career up to this point, we've operated with head coaches and systems that were primarily offensive in orientation. Karlsson's role on those teams was the be the penultimate offensive catalyst in moving the puck up the ice, participating in the counter-attack and organizing and managing offensive zone play.
There was a defensive aspect to his play in that he could move the puck quickly away from his own end, but this was not the primary focus.
Things have changed this year with Guy Boucher, who has implemented a conservative defensive structure that relies upon the team being more careful with respect to decision-making, particularly when it comes to counter-attacks and joining the rush.
I'd argue that Karlsson is playing more conservatively as well, as both the eye test and slightly lower point totals might confirm. There are a host of other factors in play for reduced points (PP effectiveness, slumps and injuries from Hoffman, Ryan and Stone early in the season) but ultimately he's playing differently this year. His ice time has also been reduced and he's playing quite frequently on the PK.
Now 37 points in 43 games is hardly a slump, and it's more than possible that he could hit his previous offensive levels in the second half.
To answer the questions of whether the system is holding him back, I would argue that previous systems may have helped with emphasizing his offensive totals but with a resulting cost on his all-around play. His current system could very well reduce his offensive contributions but I think it helps with his development into a more well-rounded defenceman. People who watched him play for Sweden will note that he played more conservatively in that structure but still led the Olympics in scoring.
Meanwhile, this "mediocre team" is holding "superstar" back could be applied to 2/3 of the league and their top player.