Mixers was viewed illegal because of the scandals and crimes that involved and use mixers upon executing their crimes, giving people the thought that using it is a crime and is illegal. But technically, it isn't. Mixers are actually helpful especially for crypto enthusiasts and people who wants to do a private transaction for their businesses. Bitcoin mixers breaks the links between your address and the other parties transactions providing you anonymity to your identity. The legality of mixers varies with how and where would people use it for.