Why is deletion mutation worse than point mutation?

Because a point mutation is the replacement of a single base with another base. Thus, if the replacement base codes for the same type amino acid, such as a hydrophobic type amino acid replacing a hydrophobic amino acid, no change to the protein coded for will occur.

If a deletion mutation occurs then the code for the protein is disrupted and the protein product is probably going to be useless to the body and may even cause a disease.