According to this, About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support , the adaptor should work.
"You can also use the Thunderbolt port of this adapter to connect an external hard drive, dock, Mac, or other device that uses a Thunderbolt or Thunderbolt 2 cable for data transfer. This adapter supports data-transfer speeds up to 10Gbps with Thunderbolt devices, and up to 20Gbps with Thunderbolt 2 devices."
However connecting two Macs directly can sometimes not work as smoothly as an external drive. As others have suggested, make a Time Machine (or "clone") backup of your old Mac and connect that drive to the new Mac for the migration, that has always been the most trouble free approach in my experience.