Although I can create waypoints on mobs inside a single player game, they do not appear in the locator bar unless I first open my world to the LAN.
To recreate: Enter a single player world. Summon a mob and make it a waypoint using /waypoint. Note that the mob’s waypoint is not visible in the bar. Open the world to the LAN. Notice that the waypoint is now visible.
The purpose of the waypoint mechanism is to allow location of entities. Because mobs can be waypoints, there is no need to be in a multiplayer world for them to be useful.
I just tested something, and found this was my mistake. I can’t mark this closed myself or I’d do so.