Are genetic mutations (such as the ability to produce lactase in adulthood) considered desired traits?

It really depends on the type of genetic mutation. You see, genetic mutations can be both good and bad for us. For example, the ability to produce lactase is a desirable thing because it helps us digest and break down lactose, which is a type of sugar found in milk. If we did not have lactase in our bodies, when we consume milk, it would result in temporary sickness since we are unable to break it down. A bad genetic mutation could simply be being born without arms - obviously it is very uncomfortable, so it is undesirable.