Don't make the mistake of confusing a mutation with a genetic defect. Someone born with thalidamide type flippers for arms isn't a mutation, they are defective from a genetic standpoint.
Dictionary:
mutation
(myū-tā'shən) pronunciation

3. Genetics.
1. A change of the DNA sequence within a gene or chromosome of an organism resulting in the creation of a new character or trait not found in the parental type.
2. The process by which such a change occurs in a chromosome, either through an alteration in the nucleotide sequence of the DNA coding for a gene or through a change in the physical arrangement of a chromosome.
3. A mutant.

The ball python in question is a mutant (def #3), a result of the event of definition #2: a rearrangement of chromosomes prior to its birth, dealing it a set of alleles that resulted in the inability to form scales, or perhaps a result of definition #1: a change in the sequence of its DNA making the proteins required for scale formation not-viable. Obviously this process could have been interrupted on any part of many pathways.

The organism is viable and stable. It could be considered "defective" because it doesn't have the protective layer over it, but neither do hairless rats, cats, or monkeys, who all do well and thrive in the comfort of a domestic environment.

I am wondering what definitions of 'mutation' and 'genetic defect' you're using that you don't think both are possibly applicable here (even interchangeably). I'm also wondering what course you took (who your teacher was) that gave you this impression...?