If a poncho has grommets or tie strings at the corners and maybe at a few spots along the edges, it will make a perfectly fine shelter. For many years as a starving student, i backpacked with a standard GI poncho as my shelter and it worked just fine, being in many ways more versatile (and cheaper) than a tent. Do carry assorted lengths of cordage and learn your knots.

It is always a good idea to keep the poncho clean and these days including a thin painter's tarp or thin space blanket solves that problem. In order to avoid pcking around excess dirt, I will use a space blanket groundsheet beneath a tent, as well. They have been surprisingly durable.