Let me ask this, purely as a hypothetical, if AVP were more experienced and better at calling plays, and Covington were better at reacting to what's happening and adjusting the defense, would it matter that Mayo is useless?
Yes, because Mayo still has a role to play and he is not fulfilling it. You simply can't just be a "CEO Coach" in the NFL. If AVP and Covington were both experienced and competent, it would definitely produce more wins, but it wouldn't ever produce more than a ~.500 team without the HC doing his job too. That is how the one-score games are won and lost.
You can point to this game being a one-score game, but it was against one of the worst teams in the league. Their "one-score losses" have been against the lowly Colts, Titans, the Snoop Huntley-led Dolphins, the Rams who are very hot-and-cold right now, and the Seahawks all the way back in week 2.
They lost to the freakin' Jaguars by 16 points and the Jets the first time by 21 points. They lost to the Texans, 49ers and Tua-led Dolphins by an average of 19 points and only because most of those teams let their feet off the gas.
So they are basically just barely managing one-score losses against the worst teams in the league, losing big instead to some of them, and occasionally barely managing a one-score win against some of them (the 16 point Bears win being the one exception), and then being utterly trounced by the actual good teams.
But I guess if you ask Mayo, "if you get rid of all the losses by more than one score, they have been in all the games til the end" right?