A single bad seamstress causing bad clothing to be delivered to customers is a process failure, the same way no single engineer should be responsible for Saleforce's production systems going down. And in most ways, implementing quality control on your product is a lot easier than controlling the personal life of your spokespeople.
Any company that's paying $10M to a spokesperson will have (maybe I should say "should have") a lot of employees dedicated to that spokesperson and the campaign. They might have a clawback clause if the spokesperson shows bad judgement, and can also get some additional good marketing if they are proactive in distancing themselves from their Kanye-esque spokesperson.
So I guess, like you say, there is a quality control process regardless.
I think the larger issues are probably the decisions of the pre-seamstress process employees. Bad choices in materials or cost-savings measures can turn off customers.
This kind of makes Kim K a safer play than a bad seamstress. As unless you go full Kanye a spokesmodel isn't going to tank a company.