It is a fair way to think of it. A skin of moss over the ground does much of what a layer of bark or gravel does, except that it is alive, renews itself, and looks like a small green landscape rather than a dressing. In a shady bed, moss is mulch that gardens itself.
What mulch does, and how moss matches it
Mulch earns its keep in a few plain ways, and moss covers most of them. It keeps moisture in, since a moss layer slows evaporation from the soil beneath and holds water like a sponge after rain. It suppresses weeds, because a dense carpet leaves bare ground little chance to seed. It buffers temperature, keeping roots cooler in heat and a touch more sheltered in cold. And it shields the surface from the battering of heavy rain, which on bare soil drives compaction and run-off. A bark or gravel mulch does these things passively as it sits there; moss does them while quietly growing.
Where it shines, and where it does not
The catch is the same one that governs moss everywhere: it wants shade and damp. As mulch it comes into its own in shaded borders, around the feet of ferns, hostas and woodland perennials, beneath trees and shrubs, and over the soil of containers and bonsai, where it keeps the root zone cool and finished. It is no use as mulch on a hot, sunny vegetable bed, where bark, straw or compost serve far better. So it suits the shady, ornamental parts of a garden and leaves the productive sunny ones to conventional materials.
How it differs from bark and gravel
Two differences are worth weighing. The first is feeding: a bark or compost mulch rots down over time and improves the soil, whereas moss adds very little nutrient and does not break down into the ground, so it protects the soil without enriching it. The second cuts in moss's favour: a conventional mulch needs topping up every year or two as it rots or scatters, while an established moss carpet renews itself and, if anything, thickens with age. Moss also takes nothing from the soil, having no roots to compete with the plants it surrounds.
The catch with seedlings
One genuine drawback follows from moss being such good ground cover: it is as happy to smother your seeds as the weeds you wanted gone. A dense moss layer makes a poor seedbed, and it can also hold damp against the soft stems of low plants in a wet spell. So use moss as mulch around established plants, and keep it off ground where you intend to sow or where you are raising seedlings. Treated that way, around the things already growing, it is one of the lowest-effort, longest-lasting mulches there is.
Putting it down
You establish mulch moss exactly as you would any moss: clear and firm the ground, press in transplanted patches or paint on a slurry, and keep it damp while it grips. The creeping carpet-forming kinds knit fastest into continuous cover, and the ground cover guide and the growing guide both cover the method in full. Once down, it asks for nothing but the autumn leaf-clearing every moss surface wants.