Positive mutations tend to get ignored. If someone is 10% smarter or stronger than the general population due to a mutation, how would we notice that the mutation existed? Furthermore, positive mutations eventually become the norm in a population, so they eventually become the status quo.
However, a negative mutation is easily identifiable because of the unfortunate effects of such a mutation.
I agree that positive mutations might be hard to spot.
But it's not at all clear that there have been any. There have been advances in nutrition and healthcare, that have enabled us to live longer than people did a century ago, but 120 years as an approximate age limit was written in the Bible 3500 years ago. If anything we still seem to be living shorter lives.
There have been advances in human knowledge, but as far as true intellectual capacity, it doesn't appear that man has evolved in some time.
The changes that we do have, such as the shortening of the jaw resulting in wisdom teeth, do not appear to be a positive, but rather a negative health impact that must be overcome with modern medicine.
The likelihood that a random mutation actually improves functionality is incredibly small. Most mutations are harmeful. It still looks like man is de-evolving. And that the negative genetic mutation load vastly outweights the positive.