"The quality of this pair of shoes is not very good."
Yes it is normal to have 的 between the two to show the relationship, but in colloquial Chinese it is acceptable to omit 的, as the word 質量 comes after 鞋子, so people would understand that you are talking about the quality of the shoes anyways. I usually include 的, and the reason for skipping it may be a slip of the tongue.
This probably should not be viewed as colloquial or a slip of tongue, but a valid syntax structure. It is allowed in Chinese to extract the principle noun phrase and make it a topic phrase at the very beginning of the sentence.
As @DariusLee3 explained above having 的 is also correct. We were only discussing that without 的 it is also a viable syntax.
My question exactly!