I don't think we can blame Marner for Matthews inconsistency and puzzling stagnation. To me, it all began once Matthews started acting like a celebrity instead of a hockey player. Overnight he went from a clean-shaven, high-tempo hockey player to a model with his hipster mustache, silly Disney apparel, high fashion outfits, tat sleeves, and following Bieber around like a roadie.
Of course that coincided with the Toronto Maple Leafs completely cowering to him, treating him like royalty, giving him a blank check, and agreeing to his short term demands. To this day, it still seems like everyone in the city of Toronto -- or connected to the organization -- is terrified to even question him, demand more, or call him out on a career's worth of subpar playoff performances.
If you look at his body language on the ice, he's lumbering and lethargic at times, lacks the same intensity that he showed as a rookie, and has zero killer instinct. He appears to just want to go out there, get open space and fire bullets. That's great and all during the regular season but it doesn't fly in the playoffs. I think that's all on him and has nothing to do with Marner.
I think we'll all see where Matthews priorities lie when it's time to sign his next deal. If he isn't willing to take a lot less so the Leafs can ice an adequate team around him, then his main priority is his contract and not winning a Cup.