It wouldn't have been a nuclear winter. The nuclear winter theory posited that a large nuclear exchange, particularly one that tried to "dig up" buried missiles with multiple strikes, would send a great deal of the Earth into the Sky, reducing the amount of sunlight that reached the Earth. There was no nuclear exchange on Endor, just a big hypermatter-powered ball of metal blowing up in orbit. It would have been more like the K-T extinction event, but much more severe -- as big pieces of Death Star plowed through Endor's atmosphere, they would heat the air to the point where they generated radioactive bow shocks, set all the trees on fire, irradiated the Ewoks, then hit the planet's surface with many many gigatons of force. That obviously didn't happen, and there are a couple of theories as to why.