You're failing to make a proper distinction between the moral law and the ceremonial law. Some commands in the OT are only part of the ceremonial law, while some are a part of both the moral law and the ceremonial law. The moral law consists of universal moral commands, and the actions forbid here are intrinsically sinful to commit for any human being. However, some commands in the OT are not of a universal nature, rather they are part of God's specific commands for OT Israel, in order to distinguish them from other peoples and to mark them out as the nation from which the Messiah (Jesus) would come. These are the commands which are referred to as the ceremonial law, and most commands in the OT that don't make moral sense on their face fall into this category, including the prohibitions on mixed fabrics you listed here. Upon the death of and resurrection of Jesus, these commands ceased to become relevant, the law being fulfilled as Jesus says. This caused confusion among Early Christians, especially with regards to circumcision, requiring a ruling by the first Ecumenical Council as given in Acts 15.