You should think of this phrase in the context of a graph. The demand is flat means that there is no growth in the number of people who want fashion accessories. If the demand were rising then there would be a spike on the graph.

"demand is relatively flat ，需求相对稳定 整句话的意思是 时尚装饰物的市场需求已经相对稳定 ，公司应该考虑开发新的利基市场 需求是一定价格水平下，消费者愿意并且能够购买的商品数量 需求的的相对稳定就是指 一定价格水平下，消费者愿意并且能够购买的商品数量的稳定 hope it would help