Is black plastic or landscape fabric a good mulch?
Black plastic can be used to heat up an area (such as for sweet potatoes or pumpkins) but should not be left in place as it prevents moisture from entering the soil and otherwise interferes with soil health. Roll or fold it and reuse the plastic year after year.
Landscape fabric, or so-called geotextiles, are not a substitute for mulch. They should not be used in garden beds, though they may have a role as a weed-block beneath gravel of pathways or patios, for instance.