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

Here is Why You Need An Adapter

The outlet and plug type used in Nepal are C, D and M and in Mongolia are C and E. This is how they look like.

Mongolia

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

Type C

Type E

Nepal

In Nepal, outlets of type C, D and M are in use.

Type C

Type D

Type M