Its an interesting topic this one. The theory that clones degrade in some way through successions is thin. But not at all impossible.
Clones are classified as asexual reproduction which can occur naturally through organisms that intentionally produce identicle organisms of themselves through mitosis. Or through practices such as vegetative propagation which is known as cloning for cannabis.
The thoery behind succesive cloning degradation, is through genetic mutations. Although it has been shown that several generations of mutations can have an effect on genetic variances. It is a very small effect.
I believe the differences people see are more related to phenotype plasticity, where through environmental conditioning, changes the phenotype expressions of a certain plant. Which then carries on this trait through its next asexual offspring. However, this shift does not automatically mean a degradation of said genetics. Infact, it would imply the opposite, however it can of course promote negative phentype expressions that are not valued by a grower. Such as less yield.
There is also the possibility that pathological diseases can cause problems that stay with the plant through successions. This may just as likely be the cause.
One thing seems clear to though, is that successive cloning does not automatically degrade the genetics and if the original or successive genetics are well looked after, is a high possibility it will perform as well as its original mother. There are many people who have cloned the same genetics for decades and some species of plants have been carried over for centuries. Many organisms, do this naturally without third party intervention. Which is a natural process of these organisms. So its hard to believe that this is a genetic aspect, but likely more related to phenotype expression or pathological diseases.