To use a more precise definition: "Genocide is intentional action to destroy a people (usually defined as an ethnic, national, racial, or religious group) in whole or in part. The hybrid word "genocide" is a combination of the Greek word γένος ("race, people") and the Latin suffix -caedo ("act of killing")"

She absolutely went in for mass slaughter, I'm not saying she didn't. But she was not trying to intentionally wipe out the entire Westerosi people (or for that matter, the entire race of any of her conquered cities), so it was not a genocide.