Not cardboard per se, but perhaps you might want to look at Foamcore board at the arts&crafts store. This is like a cardboard, with a layer of dense foam sandwiched between two thin paper outer layers. Thickness is from 1/8 (no good for this application) up to 1/2 inch (excellent). It cuts with craft knives & parts pin into it easily. The foamcore is not affected by water -- so it can be a good option as a base for building plaster groundwork. It too will be lighter than wood of the same thickness.
You can even make buildings & structures out of Foamcore. I discovered the stuff when my son was in 4th grade & needed to make a castle for school. Cut out the parts, pinned & glued them together & sprayed Fleckstone paint on them. I've since seen it used in making buildings in dioramas. Make the building (or part of it), cover with spackle, and paint.