There are many different expressions depending on the location of the mountains and also the type of people living there. For instance, in America, we call families who live in the rural areas of the mountains "hillbillies." Men who trap animals and live from the land are called, "mountain men." Those people who live in the highlands of Scotland are referred to as "highlanders." They call the people who live in the southern part of Scotland, "lowlanders."

I'm not sure if we generally have one collective noun for people living on mountains or hills. Hill people. Hill dwellers. Mountain dwellers. Mountain folk. The northern half of Scotland is, I believe, known as The Highlands. Therefore someone who comes from that region may be known as a highlander.