So since mxtx has said that in novel canon nie huaisang is eventually going to end up being chief cultivator we can presume the headshaker thing, while based on his existing personality, was in fact an act and he is way more competent at his job than he's let on. And with Jin Guangyao dead and gone he has no reason to put on that act anymore.

Imagine being an Nie cultivator. Your flopppy incompetent sect leader comes home from traumatic events after it turns out his older brother's sworn brother- who's been over to your sect basically every other week for years now- is the one that killed said brother and you're fully prepared for tears and dramatics. And while you definitley don't like lianfang-zun since it just came out he killed your last (way more competent) sect leader he was sorta the one holding the sect together so you're bracing yourself for the worst, and instead he's... stoic? Paperwork is suddenly getting done well and on time without help. He's not wailing during negotiations anymore and you're getting actual good trade deals for the first time in over a decade. He answers questions now? With something other than "I don't know i don't know i really don't know!"

And even if you had none of the revelations wei wuxian had in guanyin temple, slowly the realization creeps up on you that all his earlier embarassing antics that cost your family their comfortable livelihoods and your sect their good name was on purpose. And from there it's not that hard to put two and two together. The rest of the cultivation world assumes the shock of jin guangyao's betrayal has finally forced the headshaker to grow up a little but you have worked side by side with him for a decade, probably even watched him grow up under his brother's rule. You know better.

How do you feel at that point? What can you even do? He's your sect leader. And he's finally doing his job, things are looking up, shouldn't you feel relieved? It' not like his incompetence really caused any deaths among the nie, just wiped out generational wealth and reputation that will take years and years to build back up. Is it really reasonable to only start hating someone for something when they've stopped doing it? Thereby proving that they were capable of stopping all along?