The "mounds" were designed by Arthur Edwin Bye in the 1970s.
He enhanced the pre-existing character of a site through the subtraction or addition of plants, rocks, or grading, resulting in a landscape that presented natural features with greater intensity and unity. This deferential aesthetic was grounded in Bye’s emphasis on the human experience of natural systems. (source)
Pioneer Landscape Architect in Naturalism
"Athur Edwin Bye, a landscape architect whose public and private garden designs strove for a naturalism so artful that students said he knew how to make the snow fall where he wanted" (source)
There used to be wooden benches surrounding the mounds but NYC Parks has never repaired them since the 1970s.
-- Jonathan Hilburg
Arthur Edwin (A.E.) Bye's stone-and-grass mounds ... have served as gathering spaces, shaded picnic areas, and interactive climbing features since their installation. ... A $10.5 million renovation and a “grand new entrance” to the park would scrap that.
“Despite community outcry, the Parks Department is proceeding with plans to cut 58 park trees, and to bulldoze popular landscape features in the historic park,”
“In addition to removing scores of trees, the Parks Department plan would also demolish a picnic area and rolling landscape mounds that are popular with neighborhood families. In what neighbors see as a scandalous act of social engineering, the Parks plan would relocate the leafy picnic grounds to a new, and more exposed site across the street from an existing NYCHA building, and away from the planned luxury high-rise.”
Bye’s subtle designs consciously enhanced the natural form of the landscape through the addition or subtraction of existing natural features and the physical molding of earth. He was an early proponent of an ecological approach to design, his projects often incorporating natural processes and environmental remediation strategies into their designs.