It is usually said that most of the new mutations are deleterious. The proper function of a gene is based on the right gene products in the right ratio. When a mutation occurs, it changes the gene product or deletes the gene product or changes the ratio of the gene products. These three changes brought about by a mutation that can easily destroy the existing function of the gene.
A mutation can cause change, which can be an improvement or a worsening of the existing system. Usually, beneficial mutations are rare. A beneficial mutation would have to improve the existing system in the matter of efficiency or speed or in some other way.
Since most mutations are spontaneous, it’s difficult for a mutation to be beneficial.
Hence, a mutation is usually deleterious and rarely beneficial.