That is an incorrect definition of evolution. The most basic, modern definition is "a change over time of gene variation representation in populations". Part of that gene variation comes about due to mutations and over longer periods of time, those variations can lead to speciation. Evolution isn't about
improvement. Beetles, nematodes and bacteria are tremendously successful beings for example.
You really need to read up the subject. Some mutations cause genetic diseases and conditions. I recently learned I have a variant gene called HLA-B27 which causes the condition Ankylosing spondylitis, because I didn't get it diagnosed soon enough, I've developed a hunch back and can barely turn my head. That, however, is a genetic condition, not the sort of mutations we're talking about with regard to evolution.
Perhaps this graphic will help.
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