There is a lot happening in and under that workhorse layer.
Of course, there can be too much of a good thing.
Keep the mulch at least several inches away from tree and shrub trunks.
It is worth mentioning these days that storing that carbon of organic matter in the soil not only improves the quality of the soil, but also does not release that carbon into the atmosphere, protecting it from the pollution we make by burning that carbon (those leaves or grasses and sawdust).