http://vww6ybaluuo4gfuuf6zvlt23hvg2pcfayco5iiwhv3plo6o7bewivoid.onion/en/about-us/press/fbi-seizes-anonymous-remailer
So yes, criminals could in theory use mixmaster, but they already have better options, and it seems unlikely that taking mixmaster away from the world will stop them from doing bad things. At the same time, mixmaster and other privacy measures can fight identity theft, physical crimes like stalking, and so on.