It's hard to tell what you are suggesting. Using plaster and cloth or paper over a shape is a good way to make mountains and other terrain features, but the finish is arbitrary.
On the other hand, as Don suggests, objects can be cast in a mold with certain limitations. In that case, the finish is a product of the inner surfaces of the mold.
I've made simple objests like box car bodies or flat cars that way. Plaster holds fine detail well. Solving the mold "draft" problem can be achieved if you engineer the model in muiltiple pieces. An example would be a building. You can make a mold with brick detail, out of RTV (room temperature vulcanizing rubber).
Cast a supply of wall blanks and individually cut doors and windows.
If you can figure a way to put together a car body, please share. Something like a shipping container would be really doable.