Mutations (changes in sequences) should not primarily be thought of as "bad" or "good", but rather simply as changes and a source of genetic and phenotypic diversity on which evolution by natural selection can occur. Natural selection ultimately determines the long-term fate of mutations.
EX: The pepper moth example, where there was a mutation in there cold that allowed some to blend in with the trees. Due to that mutation, the moths with a darker color were able not being eaten by the birds as the other moths were.