I’ve not recorded the results on joining varied quality cords, but would recollect that after couple joins, decent cords with harsh ones, the additional joins became harsh. (Easy to cheese around that by turning all below decent fur/leather to bandages first)
It could run similar calculations on the cord quality (I believe it uses COMMON as the skill) as repairing a fur garment with lower quality fur. E.g. repairing decent quality bear fur coat with harsh bear fur, after fixing it enough, namely 50%+ is of lower grade, the garments grade changes from decent “bear fur coat” to “harsh bear fur coat”