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

Here is Why You Need An Adapter

The outlet and plug type used in Tuvalu is I 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

Tuvalu

In Tuvalu, outlets of type I are in use.

Type I