What is the chance of mutant sperm being able to fertilize egg, where the baby born will have genetic defect/disease?

Probably does happen: This is less likely to happen than fertilization by a healthy sperm. One of the features of male reproduction that differs from that of the female is the screening process created by sperm function. Fertilization is achieved by the healthier sperm that swims better/faster & gets there first. Could that include faulty genetic information ? Yes. Yet it couldn't be bad enough to disable the sperm.