I disagree about Shula being mostly to blame. I think he is a big part of it, but you can't put the blame entirely on him
Marino simply didn't get it done in the playoffs. The Dolphins got to the playoffs often enough to leave very little doubt about that.
He was also notorious for audibling out of running plays. The Dolphins may not have had great RBs or a great running game, but they were able to run the ball despite popular opinion now-a-days. Higgs got it done when he was there.
The D coordinator is one of the biggest reasons, but Marino always gets a pass on his Playoff play because of all those gaudy career passing stats