It only takes a few minutes to fix this problem, and your trees will be much better off. Put on your work gloves and pull the mulch away from the trunk of the tree. The profile of the mulch ring should look like a donut, not a volcano. Right at the trunk, you should be able to see a narrow ring of soil. Mulch should be no more than 4" deep in a ring surrounding, but not touching, the trunk.
If mulch is too deep, this is bad for the roots. Most of a tree's roots are in the top 12-18" of soil, because they need oxygen. Spreading out the mulch ring wider is fine. Having a mulch ring helps to retain some moisture in the soil during drier weather. It also adds nutrients as the mulch decomposes and encourages whoever mows the lawn to stay clear of the tree trunk. Lawn mower wounds to a tree trunk can significantly shorten the life of a tree.
This Facebook page is a good source for tree care tips. Morton Arboretum also has a good page on the subject.