They Bungled the Mad Queen Arc
This was season 5 Walter White showing up at the beginning of season 2.
Daenerys was always Messianic. It makes sense she's threatened by Snow, a brave, humble, natural leader of men who has a stronger claim to the throne and literally came back from the dead. This fuels some anxiety, but Messianic people do not suddenly have a full-blown crisis of confidence. So northerners love him more than they love her? They're northerners. She understands the insular culture and affection for a favorite son. She's managed to win over Western elites who were her enemies (Tyrion, Varys, Jorah). Cersei has the irrational confidence that's seen in dictators: her city is on fire, but she still feels her soldiers will turn it around.
This notion that Dany's curb-stomp was presaged is nonsense. People cite past dialog where she threatens to burn cities to the ground -- "that's her impulse" -- but then she never does and celebrates herself as a "breaker of chains." In the seventh season, there are at least two incidents where she says she's NOT going to reduce King's Landing to a pile of ash, yet these comments are also interpreted as "foreshadowing." So when she says she's going to burn a city (and doesn't) it shows her "real" state of mind. When she says she's not, it also somehow shows her state of mind. OK.
This idea that the violence was "always there" plays fast and loose. The same goes for the idea that people made excuses for her. The vast majority of characters in the series have blood on their hands, which is constantly rationalized. Varys, who in the latest episode says he doesn't know how the Targaryen girl's coin has landed, once tried to have her assassinated. He confesses to doing other fucked up things. Tyrion strangled a whore and murdered his defenseless father. He could've just left, but chose to kill (he also helped set thousands of young conscripts ablaze, and served the evil King Joffrey). Jorah Mormont took slaves, spied on Dany, and helped with the assassination attempt. Jaime pushed a kid out a tower. The Hound has slaughtered innocents. And so on.
One thing about GoT is that almost every character gets a chance for redemption, including people who have said and done evil things. Inasmuch as it's obvious Daenerys is going to break bad, everyone aligned with her is morally responsible for propping up a known monster.
But no, the signs for THIS massacre were not there. In terms of this universe, she was a relatively benevolent dictator. Her Messianic is constantly confirmed by fortune and magic. Her refusal to slaughter innocents fed her narcissism.
What has always been lurking in the background is that Targaryens often suffer from madness due to inbreeding. The Mad King was also relatively benevolent early life, but his descent into cruelty was comparatively slow. He had tongues cut out and committed atrocities before demanding "burn them all." Where was Dany's paranoia? Dictators believe in themselves, but they don't trust the people around them.
Tyrion told Varys of Jon's parentage before his Queen -- an admitted mistake. He's been awful at predicting what will happen. His brother tried to sneak past their supply lines. What was the consequence? "Your next mistake will be your last." Why not accuse him of putting his family ahead of his responsibilities as Hand and have him executed?
In some respects she resents and fears Jon Snow, but she's willing to put those feelings aside if he returns her affection. What the fuck? The story line easily could've gone the opposite way: He's still in love with her but she no longer cares for him because he's a threat to what she values more: unrivaled power.
With the bells ringing, the city had in effect bent the knee. Yet she flips her lid and goes off. It was just stupid and undercut any real moral ambiguity. Taking a city involves deaths of innocents. This was a war of choice, not necessity. Instead they gave her Hitler mustache and she just needlessly slaughtered people. She just went cartoonishly mad.
Hell, they could've had some townsboy throw a spear and stick her "child." She incinerates the kid, then gets a taste for it. He's a stand-in for the regular people who will always take shots at her because she's not properly feared.