I create mostly logo designs using Inkscape on Linux but I cannot find a way to mockup my designs with an open source software. Are there any open source programs to apply vector (or raster) designs into mockups for showcasing designs? Similar to what Photoshop calls 'Smart Objects.'
Of course, depends on what do you mean by showcasing your designs and what those designs are.
My option would be, create your mockup.
If there are no macros or smart objects on Gimp to reproduce shades and perspectives, my option would be to generate a full 3D object.
Prepare some 3D models, for flat logos, for business cards, for a flyer on Blender.
Then you only need to change the base texture and generate a new render with the new design.
Many programs open PSDs, for ex. GIMP, Krita, Paint.NET, Serif Affinity Photo & -Design(not free, only low cost), but the result is partial. They do not interpret properly Photoshop's advanced side such as vector masks, smart filters and layer styles. Normal raster layers and layer blending modes are interpreted ok as long as the program knows the used color system and the blending mode.
That's unfortunate, because many high quality PSD mockups are based on PS's advanced features. A good mockup can radically reduce the needed not-so-creative work, you get right bendings, perspective, lights&shadows and texture overlay virtually with no work. Add only your colors, texts and the design drawing, tweak the effect strengths as you like.
- get Photoshop - even for a short time, it's now rented, you cannot buy it except pre-CC versions (poor SVG support) as second hand stuff. If you haven't already used it, you have a short fully free trial period.
- test the available mockups "what you get in GIMP or other available software which reads PSDs". Rebuild the missing things such as the perspective and light with program's own methods
- start from scratch. That's a long term solution and will be a substantial skill level jump, too. Usable mockups can be built without smart filters and layer styles.