Dogwood tree problem
Dogwoods are excellent native trees that normally appear in a forest understory or edge. So, it's good for it to have an organic mulch that decomposes and makes the soil around it more organic, like a forest soil.
Mulch should never be piled on the trunk of a tree, however. (That would not happen in nature.) It should be pulled back a few inches so that the trunk and the flare, where the trunk spreads out and goes into the ground, are visible. Mulch piled on any plants--shrubs, perennials--holds moisture in and can encourage disease. It can also provide cover for boring insects.
Since we can't see the bottom of the trunk in the photo, we can't see the flare. When you pull back the mulch, be sure that the flare is visible. It's easy for a tree to be planted too deeply, but this will slowly kill a tree.
Newly planted trees and other plants need to be watered to supplement rainfall for at least the first 2 years as they get their roots established. Usually that means being aware of weather from spring to fall. A rain gauge is very helpful. Do not water on a schedule, as that could drown a tree in a wet season. The brown tips and edges of the leaves in your photo point to over-watering.
The soil should be as moist at a wrung-out sponge (the surface will dry--that's ok). Plants like about 1" of rainfall a week. If the soil is dry about 1/2" down, be sure to water.
Also, new trees do not need fertilizing except a liquid fertilizer when they are planted.
Dogwoods are the first trees in fall to change color. Some of the color change you see may be that.
Here's some good information that may give you some more ideas: https://extension.umd.edu/hgic/trees-and-shrubs/planting-process Also read the page on Post Planting Care.