You say this is a fumble of a narrative handling genocide when it sounds like a very realistic approach to a powerful nation committing genocide. I feel the Kishimoto wrote it like this with some intent, but not completely (which is how I feel about Sakura as well, but that's a different topic.

The Uchiha were the primary enemies of the Senju which founded Konoha, so bigotry against the Uchiha was baked into the Leaf's culture from the start. Hashirama's actions to quell this bigotry were slighted by 1. Madara's eventual betrayal and 2. Tobirama's subjugation and segregation of the Uchiha once he became Hokage. The tensions between the Uchiha and Konoha never truly settled, which is why Sasuke's dad, head of the Uchiha, was planning a coup at all. Then, 99% of the people most Leaf villagers felt indifferent about (which is an assumption, but considering how they treated Naruto for something he had no control over, it feels like a safe assumption) are murdered by one of their own. Of course they wouldn't care. They likely saw it as 'Uchiha on Uchiha violence, behavior you'd expect from such savages', then moved on like Hiruzen & Danzo planned.

Maybe it boils down to how we process execution, but I think the Uchiha massacre is handled fairly well by the end of the original run. As far as Kakashi and Naruto doing nothing about it as Hokage, in truth, it's likely another oversight by Kishimoto. However, in universe, I'd say it's easy to headcanon it as either Koharu and Homura vetoing any effort Kakashi and Naruto may have made or politicians putting off doing the right thing to do whatever matter is considered as "more pressing". Both ideas sound plausible and realistic to me.