なんでかはよくわからいけど、volumesでリソース?を定義してそれをマウントしようとするとうまくいかなかったので、直接書いたらいけた
Docker for WindowsをWSLで使ってる

 db:
    image: mariadb:latest
    environment:
      MYSQL_ROOT_PASSWORD: password123
    networks:
      mynet:
        ipv4_address: 172.21.0.2
    volumes:
      - mysql-vol:/var/lib/mysql
volumes:
  wp-vol:
    driver_opts: 
      type: none
      device: /mnt/c/container/Docker/wordpress/volumes/html
      o: bind
  mysql-vol:
    driver_opts: 
      type: none
      device: /mnt/c/container/Docker/wordpress/volumes/data
      o: bind
以下のように変更
  db:
    image: mariadb:latest
    environment:
      MYSQL_ROOT_PASSWORD: password123
    networks:
      mynet:
        ipv4_address: 172.21.0.2
    volumes:
      - /mnt/c/container/Docker/wordpress/volumes/data:/var/lib/mysql
# volumes:
  # wp-vol:
    # driver_opts: 
      # type: none
      # device: /mnt/c/container/Docker/wordpress/volumes/html
      # o: bind
  # mysql-vol:
    # driver_opts: 
      # type: none
      # device: /mnt/c/container/Docker/wordpress/volumes/data
      # o: bind
