I used a GI-type poncho tarp and relatively little else for about 35 years in the Sierra and the North Cascades. I had a tent (8 lb) if we anticipated camping on snow.

If the weather appeared to be nice, I would just lay on top of the poncho and if it started to rain I would just pull it over my bag and head. That would work OK, sorta. The bag would get a bit wet from condensation before dawn and would need to be aired out the following day.

If the weather was windy and rainy/sleety/snowy then I would pitch the poncho low on the windward side and hope that the wind didn't shift during the night. It usually did though and if I was awake I would be out dancing around in the rain in my underwear revising the pitch. Glad they didn't have cell phone video cams in those days.

I used a piece of poncho fabric about 25" x 80" as a ground cloth. Not light by today's standards but it was cheap.

Needless to say, I now prefer a tent. I use a BA Fly Creek with a sil-nylon awning added to the rain-fly to keep water out of the tent during rainy exits. The Fly Creek weighs less than did my old poncho setup and it is more weather-tight. Like Ringtail, I prefer the quality of life under a tent now more than that under a tarp.