Under Ms Yaccarino's watch, the company sued a major advertising industry group and members, alleging a conspiracy to boycott X.
The industry group shuttered shortly after the lawsuit was filed.
The platform's tone changed significantly during the course of Yaccarino's tenure.
Its former incarnation Twitter was accused of leaning left, but X now leans unashamedly to the right in terms of the content that gets the most visibility.
Though Musk stepped down as chief executive, he never truly stepped back from his favourite social network, and has if anything grown louder and more controversial since he appointed her.
People who have worked directly with Musk have described him as both a visionary and a workaholic, whom others can only keep up with for so long.
"Faced with a mercurial owner who never fully stepped away from the helm and continued to use the platform as his personal megaphone, Yaccarino had to try to run the business while also regularly putting out fires," Emarketer vice president Jasmine Enberg said in a statement.
She said her firm expected X's ad business to return to growth this year, after more than halving after Musk's takeover.
But she added: "The reasons for X's ad recovery are complicated, and Yaccarino was unable to restore the platform's reputation among advertisers."