Unfortunately, there aren't any mixers for the USDT stablecoin, However, you can mix the coin using Instant exchanges and convert to XMR.
I would recommend to you https://exch.cx/ as that will give you the best possible rates as compared to other instant exchanges. However, sometimes they are short in reserves for XMR.


You do not need to mix your XMR. The XMR which you have cannot be traced as to where they come from and where they are deposited. That is why we call XMR, the privacy coin.