It's a question of what they were foreshadowing. Obviously, they've hinted at Dany being harsh with her enemies. But massacring thousands of unarmed men, women, and children? That is very out of character for Dany, and it was presented in such a way that she literally went crazy, because the city had surrendered. There was no tactical advantage here, where there had always been one before. Obviously it came off this way because it was so rushed. If it had been given time, and she transformed into an evil Dragon Queen after taking the crown... that'd be another story.
So IMO, you saying...' oh it's been highly hinted that Dany would become a Mad Queen'.... I say....ehhh sort of. It hinted that Dany would be a tyrannical ruler... not a homicidal maniac for absolutely no reason. Being quick to anger or having a large temper is one thing.... indiscriminately killing whoever you see is something else.
If anything, they went out of their way to present Dany as a hero as deliberate misdirection to this moment. Dany is a liberator... a champion of the common man. She fought tirelessly to take King's Landing without causing unnecessary death, only to turn at the flip of a hat... and deliberately kill innocent people, for no other reason than pure blood lust. Was that foreshadowed? If so... they did a poor job of it, IMO.