You Do Need power plug adapter while traveling from São Tomé and Príncipe to Mongolia.

Here is Why You Need An Adapter

The outlet and plug type used in Mongolia are C and E and in São Tomé and Príncipe are C and F. This is how they look like.

São Tomé and Príncipe

In São Tomé and Príncipe, outlets of type C and F are in use.

Type C

Type F

Mongolia

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

Type C

Type E