Yes it was. Here is a link to the Pulitzer Award-winning stories by David Barstow, Susanne Craig and Russ Buettner of The New York Times. Mary Trump contributed tax documents to help them write these stories.

"For an exhaustive 18-month investigation of President Donald Trump’s finances that debunked his claims of self-made wealth and revealed a business empire riddled with tax dodges. (Moved by the Board from the Investigative Reporting category, where it was also entered.)"

Once she'd participated in these, and when the whole thing with children taken from the families at the border began, she decided it was time to speak out.