It doesn't make sense to introduce "dirty" fiat money into the cryptocurrency ecosystem in order to clean it. There is so much friction between fiat currency and cryptocurrency, and there isn't much volume in the markets to support large scale money laundering.

There are good ways to launder cryptocurrency itself (e.g. money you receive from the DNMs). Using VPN and the TOR browser and mixing your bitcoins is one way. Strong privacy coins like Monero are another method. But it's important to note in all cases that organizations like INTERPOL and the FBI are privy to much better blockchain analysis tools than the rest of us. There may be various privacy exploits that we don't know about yet. TOR and VPNs can be compromised. You never know. As someone mentioned, if you need to launder fiat money, there are better-established, more liquid, and safer methods to do it.