Is a mild elevation of thyroid hormone harmful?

It Could Be. One thyroid hormone lab result is a proverbial window in time. No way to know without following up on that test. It could be normal , the same, lower, or higher when checked again in the context of other thyroid tests, including thyroid antibodies.
Depends. An abnormal mild elevation over time could be harmful. Transient elevations may be due to normal variation within the body and this could be perfectly healthy assuming the elevation is "mild". Typically a repeat test is done after a period of time, maybe a few months, to confirm or refute the "mild" elevation as being an "abnormal / unhealthy" hormone elevation.