Can it be used as a mesh for IoT (Internet of Things)?

They can be used for this purpose.

The LibreRouter is a high performance mesh that can grow without much degradation.
The LibreRouters can be several kilomters with line of sight of each other and still connect and expand the mesh.

Around them they create a WiFi hotspot so devices can connect to it. One case could be WiFi IoT devices, or IoT hubs that can use the LibreRouter’s mesh network for distributing the information with each other, to a central information hub, or through the internet.

The LibreRouter also has a set of pins that can be used to connect expansions sets to it, so for example you could connect different IoT radio devices, or Arduinos or other elements to the router that can bridge in between the WiFi space and the IoT space.

The other expansion part that you might be interested in would be the two USB2.0 ports that can be used to expand the functionality like adding an extra radio to it.