It is always a good time to mulch, but it’s an especially good time now since there is some moisture in the top couple of inches of soil. Mulch holds some of the moisture in the soil, by drastically cutting down on evaporation. It can make a huge difference in how often plants need to be watered.
The most important thing to remember about using mulch is to keep it from touching the trunk of stem of the plant. Never pile mulch up on the trunk or stem, “volcano style”, as is currently in fashion. Keep it at least a couple inches back from the trunk of a tree and about an inch or two from the stem of a shrub or annual.