We weren't shown she had gone mad until she made the decision to burn everyone. She was in mourning and deeply upset. But mad? Don't think that came across until she opened fire after the bells rang.
Yet we had Varys in this very episode outright say that while he was sure Jon wasn't the mad side of the Targ coin, Dany might be (something that they've spent seasons raising a possibility already). We then immediately afterwards saw her increasing paranoia and willingness to kill civillians. They set it up,
then she finally transgressed as the bells rang. I don't see why they would have to set it up
more beforehand by showing her wandering around talking to plants or whatever.
At some point she had to snap and take the step into doing something unjustifiably batshit. The dramatic thing was to have her take that step and break
in that moment rather than have her break beforehand and just wander in to battle with us knowing she had already definitively crossed into madness.
Between repeatedly stating that she had a 50% chance of going mad, killing her children, killing her closest friends, showing her advisors turning against her, showing her paranoia, have her repeatedly threaten to burn civillians alive and have her actually brutally murder multiple enemies up to now, I think they did enough to flag in advance what was always going to be a final
leap into the unacceptable.
Part of the show naturally had to be straining against that though because the whole twist is that she's someone whose side we're generally on. To make it super smooth they would have to make her definitively a bad guy
first, which would ruin the twist of our "hero" going bad because we'd already be against her.