Hallie:  Since the plant is relatively new, I suggest talking to the nursery where you purchased the tree. There are a number of reasons why the dogwood is not thriving.  

I doubt that overwatering is the problem. The leaf curl and brown spots are usually signs of severe drought.

I think that the soil is likely your problem. These dogwoods prefer soil with lots of organic matter which creates the lower pH value, that is an acidic soil. Clay soils tend to be more alkaline with a higher pH value. In these instances, the plant develops fewer leaves and probably little if any fruit. 

To remedy the conditions, I suggest mulching around the plant which will provide a small amount of nutrients now as the mulch breaks down. In early fall, you could work more organic material into the soil in a broad perimeter around the tree. You would want to use organic material such as peat moss or dried manure which are already broken down and will provide nutrients for the roots. Wood chips, sawdust and the like are fine too but these will take longer to break down. Adding organic matter will help the root system develop over the winter.

Another possible problem could be that the root system did not develop enough over winter to provide sufficient water for the leaves.  Again, the clay soil could be an issue if the soil amendments added by the landscapers was not wide enough for the roots to develop. 

These dogwoods do best in full sun but not where they will be baked by the afternoon heat and sun. Typically planting where they get morning sun is best and is protected from intense afternoon sun. 

Don't give up on your lovely Kousa dogwood and continue watering!

You're welcome!