No, Mr Artist, I expect you artistic spirit to die!
Mockup content is created to evaluate some other aspect of the game beside the artistic one. It can be to test the gameplay or the technology for example. Other names used for it is placeholder, graybox or prototype content.
Make it fast
The time to create mockup content need to be as short as possible. It will change many times and time is wasting if to much time and details are put into it.
Make it bad
There is always a risk of people starting to focus on the artistic side of the content. That is best solved by making the content as ugly as possible and shoot down any steps to make it more into a final production version. A gray flat shaded material is a good way to show that graphical content is mockup art.
Make it bleed
As development progress everyone adjust to the mockup content and soon it will be seen as good enough. Mockup sounds will be used and forgotten about and end up in the final product. To avoid that all mockup contents need to be easy to follow wherever it is used. Put it all in the same directory and make it easy to list all the mockup content in use so it possible to remove as real content is added.
Here are lists to content that can be used as mockup art.
- Morgan McGuire's Computer Graphics Archive
- Quick And Dirty Game Art - 2012
Mesh of Box