Mayo may improve next year, I wouldn't say he's a literal moron incapable of learning.
But that's not the issue here. The issue is, yes, he's extremely green. And many posters here (not myself included, admittingly) called it from the jump: this guy has absolutely no experience and the Patriots are in a situation that seems to necessitate experience.
I can't really think of a more difficult position for an HC - hey, follow in the footsteps of the greatest coach and the greatest QB and countless SB appearances and wins, only after said greatest coach torched everything on the way out, leaving you with a garbage roster. Oh and uh, those two greatest's cultivated an intensely critical fan base. Have fun.
I hold out hope because I want to enjoy Sundays again. I miss it. I hope that the team makes a huge leap next year. But it's a big ask for any HC - let alone a super green HC w/ an underqualified DC and an OC with schizo playcalling.