Cooper is the fifth coach in WNBA history to reach the mark.
Los Angeles sat out Candace Parker, Nneka Ogwumike and Alana Beard after securing their 15th playoff spot in 17 years Sunday.
Shoni Schimmel, Tiffany Hayes and Damiris Dantas each added 15 points for Atlanta (14-18). Sancho Lyttle had 14 points and 11 rebounds, and Dantas grabbed 12 boards to help the Dream hold a 54-28 advantage on the glass.
McCoughtry scored 15 points in the first quarter as Atlanta built a 26-11 lead.
Jantel Lavender had 14 points and Ana Dabovic added 13 for Los Angeles (14-19), which had its three-game winning streak snapped. The Sparks had won 11 of 15.