Mostly Pep, by far. He stopped, by his own admission, the beginning of a squad overhaul last summer, despite the lack of any semblance of an offseason for players to recover fully from last season. He has presided over a breakdown in the pressing game, a massive vacuum in central midfield, and a complete unraveling of the backline's discipline in defending (while continuing to insist on an insanely high line). He has sat passively on the bench as results were frittered away in the late stages, not even bothering to use most of his subsitutions with viable options on the bench with fresh legs. As the squad collapsed, along with his marriage, we have seen him engage in troubling incidents of self-harm for the entire world to witness.
He got most of the reinforcements he finally insisted on in January. After a calamitous first five minutes, Khusanov turned into our best defender and was then banished into oblivion, along with Gonzales, who had overcome some stumbles to seemingly transform our midfield into a zone of competence again. He has left Grealish and Doku on the bench for extended periods when they were fit and had played well enough to suggest that they were ready for more minutes to make consistent contributions. With all the horrible problems we had with control in the midfield for most of the season, he left a frustrated Grealish nailed to the bench. Control is what Jack does, and unfortunately, sometimes that's all he does, but why not use him to best effect when you're desperate for it instead of running the aging Gundogan and Kovacic into the ground? Pep has exacerbated most of the problems created by the incredible run of injuries this season.
Pep simply must find a new system which will bring Haaland into the action. He's being mostly wasted the way he's utilized and I can't even imagine his level of frustration. Then again, his finishing has regressed badly since his debut season for us, but maybe if he got to touch the ball every now and then it might improve.
I still believe in Pep and trust that he will incorporate new players and wave his magic wand again next season. If he doesn't, it will be time to move into a new era.