No no, you are right that most observable mutations give undesirable effects. But the genetic definition of mutation is broader than that.
When you mutate DNA there are three possibilities:
1) Nothing happens (a silent mutation). This is pretty common. DNA is very fragile is easily damaged by a host of environmental factors, such as the sun. However, one of the reasons why 99% of the population does not have cancer is that the DNA may be mutated in a place where it doesn’t matter.
2) Loss-of-function. A gene, hence protein, works less efficiently, not at all, or its interaction with other protein is severly impaired. Many genetic diseases fall in this catagory. One or several proteins are severly damged by mutations and cannot perform their vital role.
3) Gain of function. A protein is better off due to a mutation. These are less perceptible. A good example would be a person’s response to a drug. A person may have a point mutation at a drug binding site that allows the drug to be more effective. We also use genetic engineering to affect a positive mutation to receive a more desirable phenotype.
Let’s see. A flower could have a spontaneous mutation that forces it to produce more nectar. which attracts more insects for pollination.