Mockup sites are finetuned services that allow you to simply illustrate a product without having to have a lot of technical knowledge.
Somebody with good photo editor skills (and equipped with skills found in the best Photoshop courses online) can create their own designs. However, this will require them to have a template image ready and an understanding of how to skew an image to fit the template’s bends and creases.
There are some sites, like Motion Array and mockups-design.com, that cater to this type of user by offering Photoshop-ready files.