Do all genetic mutations have a negative effect?

no, mutations are just a change in what they were before. if bacteria never mutated into complex organisms, we would not exist right now

Usually not. Most are just deformities and tend to cause problems for the individual.

The effect of a mutation depends on its context.  Many mutations are beneficial in certain contexts.  For example, the mutation that results in sickle cell anemia provides increased resistance to malaria.  However, all mutations involve trade-offs.  The increased resistance to malaria in heterozygotes comes at the price of sickle cell anemia in homozygotes.  There are many more examples.

Genetic mutations can be positive, negative, or unimportant.
Mutations can allow people with cell anemia to be resistant to malaria (both good and bad).

Mutations may allow some species to survive if this mutation is advantageous. For example, if some disease came about that killed except people with purple eyes, it would be advantageous for that mutation to have occurred.

In most cases it depends on the environment and natural selection to determine what is positive or negative.

Most mutations occur in areas of the DNA that do not code for anything. So these changes do not change the phenotype.