As a big time Marner hater, his playoff stats in the aggregate were fine. That said, he got progressively less effective relative to expectations the deeper the Leafs got into each series. If the argument then is, well who helped get the Leafs deep into the playoff series, then you kind of have to look at who is paid Take Over a Playoff Series money.
Coaches played him at 1D level TOI, which is their prerogative and if I honestly look at the rest of the lineups, I don't blame them for that choice (I do blame them for pushing him beyond the point of diminishing returns, especially in the regular season usage though). Marner was tied to the hip of their top scoring 1C, which again, on paper, because of his skillset, made plenty of sense for many reasons throughout the regular season. Unfortunately, when space got tight against committed defenses, Marner's skillset and defensive-minded conservativism did not help his line break through. Extra unfortunately, he aggressively negotiated a cap hit commensurate with a 1C, for his winger position at a time when the salary cap was most painful for the team, so fan sentiment compounded negatively and his public interactions with the media rarely ever assuaged anybody's perception that he was operating on petty entitlement. There's absolutely a universe in which the Toronto fanbase is unified in eating out of his hand the charming little crumbs of whimsy he brings to the Leafs if he made some different decisions and statements about that first extension, especially after witnessing McDavid get all the love for taking a significant long-term discount to help his team succeed.
Marner and Matthews being tied to the hip regardless of team result sort of sewered both their respective playoff performances. Marner always had the puck and tried to force it to Matthews against very set defensive structures, which led to turnovers or whatever, and that's 23M of the team not scoring when they needed to be scoring 60% of the team offense. Leafs special teams shat the bed every playoffs when they could have tilted a series in the positive direction, and both units were headlined by Marner and Matthews.