Clearly governments of all stripes have done their share of environmental damage. The air and water in such different countries as Mexico, India, and China is dreadful despite their extremely varied political and economic structures. The most notorious capitalist depredations have taken place outside the borders of the capitalist countries: Americans in Brazil, the Japanese in Maylasia. However, the Soviet Union turned large areas of Eastern Europe and its own territory into toxic wastelands far worse than anything in the capitalist world.
At least in capitalist economies, countervailing economic forces can sometimes create anti-polluting pressures: real estate interests oppose nuclear plants, fishing interests oppose dams. Unilateral development projects which are not forced to take such factors into account are far more likely to damage the environment.