Hardware network links can be either point to point or point to multipoint. ppp links are point to point, ethernet is point to multipoint. tun can act as either, in your case it is acting as a point to point link. a point to multipoint interface has four addresses associated with it, specifically ip address (the address of the interface), network address, broadcast address, and netmask. A point to point link has two addresses associated with it, specifically ip address (the near address) and the point to point peer address (the far address). Since the point to point link will only work with the two addresses, the broadcast, and network addresses and the netmask do not have useful data or have flag values.
A final point tun interfaces can have mac addresses, they just don't have default mac addresses.