You Do Need power plug adapter while traveling from Cambodia to Mongolia.

Here is Why You Need An Adapter

The outlet and plug type used in Mongolia are C and E and in Cambodia are A, C and G. This is how they look like.

Cambodia

In Cambodia, outlets of type A, C and G are in use.

Type A

Type C

Type G

Mongolia

In Mongolia, outlets of type C and E are in use.

Type C

Type E