Bitcoin mixers and coin mixers in general are only designed for criminals and are only used for criminals.
How do you know this? Any proofs, bro?
Bitcoin or coin mixers are basically created not for criminals, it is intended for people who need more privacy. If there are some criminals who use this service, it doesn't mean this service is created for them but it is one of the weaknesses or negative sides of this service. Then, maybe this mixer service needs more improvement.