If this is on the Ethereum platform, then it shouldn't (probably) be a problem for ETH mixers to mix ERC20 ..? Or it is?
Unless its from scam that its needed to hide the transactions from that addres but there are many ways to hide it not only mixer.
The reason why ERC20 mixer is nonexistent probably due to the fact that there are too many ERC20 out there and considering their market capitalization nowadays is quite low. Even some of the top 20 coins don't even have a mixer I can't see the reason why all these mixers websites need to provide ERC20 mixer because to make one mixer you will require a large pool of coin to be mixed into one and the amount of money needed is also enormous.
I'am surprised that the mixer must have a reserve of its own coins ( those that the client wants to mix ) to perform such an operation.
If this is the case then mixing tokens would be really difficult with such a large number of them on market.
However, I hope that someone will come up with a way to mix ERC20 tokens ..
I'm still waiting..