No, Marner selfishly wanted to be the shooter far too often.
Honestly I think its both - which seems ridiculous but hear me out.I thought his biggest weakness is always looking to pass which made the PP far too predictable.
He needs to shoot more, IMO.
Watching Marner on the PP I see a guy who's mentally stuck, everything in him wants to pass but he also knows he needs to shoot more. What ends up happening when he's in this "in between" area mentally is this - the PP moves the puck well, PK is scrambling, puck comes to Marner in a glorious shooting opportunity, but in that split second moment he wants to pass and in just that slight delay the opportunity is gone so he passes back to the point allowing the PK to reset. Then moments later the puck comes back to Marner and knowing he needs to shoot more he fires a hopeless shot into the shinpads of a defender or hits the goalie in the crest.
Marner deserves plenty of criticism but this notion that he's incapable of working a PP because his shot is too soft is ridiculous. Sure it's not a strength but it's good enough in combination with his creativity and quickness to be effective. The problem isn't the physical tools it's all mental.