If a big company like Coinbase created a KYC mixer, I could imagine myself using it in a few limited cases. True, a KYC mixer would only have value because of the evil of state-imposed KYC requirements, but it'd still potentially be useful. (In cases where I don't want anyone to be able to link my transactions, I wouldn't consider a centralized mixer in any case, KYC or no.)
I am sure this is well established already, but exchanges already act in the same exact fashion as mixers.
When a user wants to deposit, they are given a unique address, with the possibility also to receive a new one each time they deposit for most exchanges. Then when they want to withdraw, the withdrawal is processed from a hot wallet that serves a big chunk of all the withdrawals in the exchange. So it's not obvious who requested the withdrawal to outsiders.
In fact, by looking at publicized documents such as what prosecutors wrote on Binance's indictment, many parties with unknown or even malicious intent were using the exchange for these purposes, and the exchanges even through their KYC checks, didn't push for further checks or restrictions.
So when we consider what makes a mixer good, the volume it carries is a basic factor. Given that exchanges carry a big portion of all the volume in crypto transactions, they're perfect for mixing purposes as they make it super easy for anyone's transactions to get lost among the rest. And it is a fact that big exchanges have played this role in the past. KYC is easy to bypass by rich criminals actually and they do it very often when they want to utilize premium services, just using fake data and limiting their transfers each time.
So really the issue once again here is centralization.