is lichen on trees bad
These lichen are a result rather than the cause. The lichen need sunlight and do better on the trees which are not healthy, so they are more apparent on the sick trees. A sick tree will also leak nutrients and sometimes sugars and other things which the lichens can use to grow. This causes increased growth of lichen on the sick trees. As for the use of an herbicide on the trees, it may kill the lichen, but will also kill the trees.
I was once told that if a tree has lichen growing on the bark it was a indication that the tree was stressed and was in decline and destined to die.