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

Here is Why You Need An Adapter

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

Bhutan

In Bhutan, outlets of type C, D and G are in use.

Type C

Type D

Type G

Mongolia

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

Type C

Type E