Previously I made a texture in Gimp. It’s easy to save it as a PNG image and import into Inkscape. From there it can be used as a pattern fill for objects like the buttons and background in this mock up of a smartphone app.
I used the 3D Box Tool to make some buildings for my imaginary mapping app. Then I used the Filter: Sepia and Colour Extension: Randomize.
I recommend using grids and turning on snaps to arrange objects.