HONG KONG—BYD Co., China’s leading producer of electric cars, said its sales of the cutting-edge vehicles rose sixfold during the first half of the year, following moves by Beijing to encourage green cars to tackle pollution.

Still, the jump—which brought BYD’s sales of full-electric and plug-in hybrid cars to roughly 7,600 in the six months ended June 30—wasn’t big enough to signal there will be serious mass-market demand for the pricey cars, analysts said. And the increased sales of electric cars couldn’t offset a 27%…