No. You can launder money using anything that works as a transferable, divisible store of value. You just have to be able to break it down into smaller pieces and exchange the "dirty" pieces for "clean" ones. The bitcoin is just the vehicle for the laundering, and the mixer is basically just a money laundering machine. (You can use it to exchange clean assets for other clean assets, but if you start with dirty ones, you're money laundering.)