I question where you got your information from. It certainly didn't come from a biology course taught at a university. Mutations can be beneficial, harmful, or neutral.
"The underlying genetic mechanism of evolution is random mutation, and specifically mutation that is beneficial to life. Biology textbooks in theory present positive and negative mutations to students as though these were commonplace and roughly equal in number. However, these books fail to inform students that
unequivocally positive mutations are unknown to genetics, since they have never been observed (or
are so rare as to be irrelevant).
The biology textbooks in other chapters teach that
most mutations are pathologic, or disease-causing, but they don't apply that information to evolution.
The worst diseases doctors treat today are caused by genetic mutations.
Nearly 4,000 diseases are caused by mutations in DNA.4 "The human genome contains a complete set of instructions for the production of a human being…. Genome research has already exposed errors |mutations| in these instructions that lead to heart disease, cancer, and neurological degeneration."5 These diseases are crippling, often fatal, and many of the affected pre-born infants are aborted spontaneously, i.e., they are so badly damaged they can't even survive gestation. However, the biology textbooks, when discussing mutation in evolution, only discuss
the very rare "positive" mutation, like sickle cell anemia. The fact of some 4,000 devastating genetic diseases is suppressed from publication."
- Barney Maddox, M.D.1 Stolz, M. 2006. Chairman's Corner. THR Physician Connection, 9(4):1.
2 Miller, K. and Levine, J. 1998. Biology: The Living Science. Englewood Cliffs, NJ: Prentice Hall, 271.
3 Campbell, N. et al. 1997. Biology: Concepts & Connections. Menlo Park, CA: Benjamin Cummins, 426.
4 Nora, J. et al. 1994. Medical Genetics: Principles and Practice. Philadelphia: Lea and Feliger, 3.
5 The Human Genome Project. Announcement from the University of Texas Southwestern Medical School, May 6, 1993.