As a business, you should always try and showcase products that you have made, but sometimes that isn't always possible. The use of a mock up might be helpful and cost effective if utilized in the right way and all product details are disclosed.
For a mock up to be the most effective, it needs to be as realistic looking as possible. Take a look at the yellow star in this sample, you can see the wood grain through the paint. Sometimes just playing with the opacity of your image can soften the harshness of the design and achieve this look. Other times, I prefer to use Photoshop and their "smart object" feature.
I know, I know, "Photoshop" and "Smart Objects" - those can be big, scary words for some of you, but honestly it's not nearly as hard as you think it is.