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

Here is Why You Need An Adapter

The outlet and plug type used in Sri Lanka is G 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

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G