Dany going mad queen was not a terrible idea, but it was poorly executed especially when you consider:
Dany has lost two dragons. Viserion at the frozen lake, Rhegal at the Dragonstone ambush. In each case, the show did not really give her much of a reaction. A little wincy grimace when Viserion went down, basically no reaction from her (or anyone) when Bran mentioned that the Night King had made him into a wight, and no scene whatsoever in which Dany reacted to the death of Rhegal.
These are her children we are told over and over again. As each one died, they should have shown the increasing toll this was taking on Dany (rather than focusing solely on her love story with Jon), should have shown her becoming increasingly unhinged, and then when Cersei had Missandei beheaded we could see her finally snap. As it is, the whole transformation of Dany is truncated and abrupt. It's puzzling and unbelievable.
Why didn't she seem to care about the death of her dragons? Why didn't the writers use these events to move her toward the transformation they knew they were planning for her? It's because they are not really writers. They signed on to be show runners and ended up authors.
I blame George.