Why is ozone described as hazardous at ground level but helpful in the upper atmosphere?
Because ozone is an irritating gas (for almost all creatures)
Ozone is a powerful oxidant as well as strong irritant. It causes harm to respiratory systems, irritates eyes, injures or kills plants. We live on the face of the earth (crust let's say). Therefore, ozone in aquatic environment and ozone in lithosphere are not desired.
On the other hand, in the stratosphere (10 km to 40 km above Earth) ozone is beneficial.