To be fair, Cersei was using the Mountain and a more aggressive secret police version of the "Little Birds" spy network to kill anyone who badmouthed her, especially in regards to her walk of shame.
Also in show, Cersei blamed the destruction of the Sept on Dorne (who had already murdered her daughter). Yes the nobility knew she did it, but the commoners believe it was those ambiguously brown guys who killed the beloved and innocent princess.
That said, I still think Cersei's arc at the end of the series was a result of the writers on the show realizing that fans WANTED Cersei to get her comeuppance as opposed to seeing her as a sympathetic villain (as they had written her), hence making her the "final" boss on the show and ending the White Walker threat earlier than expected.
The thing is, none of this happens in a vacuum, servants aren't braindead automatons without free will or agency. The show even points out that servants are used by the nobility to gather intel... so that means they have insight into what's going on in the castle. The common people would know, this kind of information would filter down. And even if it doesn't, Sept blows up using a metric fuckton of wildfyre (conveniently something the Lannisters used mere months before to blow up a fleet of ships), King falls from window, woman suspected of sleeping with her own brother crowns herself queen. Yeah, nothing suspicious going on here.
I stand by my point, Cersei would have been found strangled in her bed one morning and a chamber maid would be suspiciously absent from her day shift the next day. Or someone would shank that bitch while the Mountain was busy getting maggots pulled from his ear by that Not-Maester or something. People would throw a parade in the streets and rejoice about the death of the Lannister Witch.
After such an act of destruction against her own city, her own nobility and her own faith, there would be no holding back from anyone within those city walls. She would not even be able to rely on the Lannister guards, cause we can suspect that a lot of them would mourn the loss of their rather charismatic king Tommen and his even more charismatic queen. Also, how many Lannister guards died in that explosion? You'd think some of them would have friends amongst their fellow soldiers who might take this shit a tiny bit personal.
Doesn't matter how imposing the Mountain might be as a Zombie, someone would get the idea to just throw a bucket of pitch at that huge motherfucker and torch him. Or go full Red Wedding on Cersei and her cronies.
As for anyone outside King's Landing:
If the nobility knows (or even just suspects) that Cersei was involved in the explosion of the Sept (and it is very reasonable to assume so), they'd abandon her cause. Cersei wouldn't even have hostages to prevent it, cause she literally burned them already in a giant green flame. King's Landing would be cut off from any kind of supply, be it food or firewood. The city would collapse within weeks due to that simple fact alone. They already went through a major famine that could only be overcome by the help of the Tyrells, their granaries are empty.
And then the combined forces of any house, great and small, would come down towards King's Landing and demand Cersei's head on a silver plate. It worked with the mad King and that guy was in a much stronger position of power. FFS, the number of people Cersei could trust would be in the low single-digits... assuming her Not-Maester wouldn't just bail out on her after that as well. Who else is there, now that I think about it? The Mountain and that's pretty much it, right?
The strength of ASOIAF was that it kept in mind that there's powerful kings, high lords, great houses, small houses and the common people and that everyone plays their part in society.
In GoT, that got pushed aside until only a handful of people from a few of the great houses actually mattered and acted like people. Everyone else was a literal NPC, ramshackled to their base function. The Guard merely follows orders. The baker merely makes bread.
None of them get any funny ideas about recent evens that bother them and might inspire some unruly behaviour.
At best, the situation Cersei created by blowing up a large chunk of her nation's nobility might be infighting in branch families trying to rise to the top. There might be no longer the main line of Tyrells, but they sure as fuck have some couple of side lines that might try to secure power for themselves. But that didn't happen either. Some Tyrells gots blowed up, so that's the end of that. Case closed. Cersei is now GIRL BOSS or something and everyone just does what she says, cause they are NPCs without free will or agency.
So even the best case scenario is rather grim, cause it means every house is now busy with their own little power vacuum and not supplying any help against the attacks coming from Danearys and her gaggle of morons.
