I've seen people build a stone-wall raised bed around a big tree on their lawn, then fill the raised bed with mulch. Avoid mulching around trees like this. It is not safe for the trees. You may build stone walls or other structures around your trees to set them off, but do not mound up deep piles of dirt or mulch around the trunks of trees.
"Why isn't it safe?" you may ask. "Isn't mulch good?" Yes, but there's such a thing as too much of a good thing. What's the right amount to apply in this case? It is safe to use about three inches of mulch around trees. Reduce that figure as you get closer to the trunk, and do not mulch at all within six inches of the trunk.
Why is spreading too much mulch under a tree potentially harmful? Tree roots must breathe, and excessive mulching can have a smothering effect. Also, necessary water may fail to percolate down to tree roots through an excessively deep layer of mulch.