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

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are 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

Namibia

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

Type D

Type M