No. There is no way to reduce the heartbeat traffic from the MetroCluster nodes towards the MetroCluster Mediator.
By design the nodes in the MetroCluster read and write to the Mediator based mailbox disks to keep their status in sync. Hence iSCSI sessions from each Node's management IP address towards the IP Address of the Mediator are established.
Roughly ~ 64kB for every 10 seconds from each node towards each of the four Mediator mailbox disks is expected.