Its an ad. That's how they work. They discredit competitors, directly or indirectly, all the time, in all kinds of ways. This is no different from putting an actor in a white doctor's coat with a stethoscope, or a fake influencer/streamer with a gaming chair and microphone in their face, to build false crediblity. Lots of companies use subsidiaries and branding to appear to be small businesses instead of a major conglomerate corporation.
They literally just need to put tiny white text on the bottom that discloses it is an ad for Shein and not a real person, the way other advertisements do, and then this whole thing would be a total non-issue.
"digital blackface" doesn't seem to be a problem when McDonalds or the NFL or alcohol companies or car companies or drug companies or anyone else does it.