Is it correct to implement a custom NgbDateParserFormatter to change the format of the input value on NgbInputDatepicker?

As of today implementing a custom NgbDateParserFormatter is the best way to go. So yes, it is a correct way.

In the future we might have a more sophisticated implementation of the NgbDateParserFormatter where you will be able to just pass a desired format (ex. yyyy-MM-dd). Adding this feature will depend on user's interest.

You might also check some more background info in https://github.com/ng-bootstrap/ng-bootstrap/issues/754#issuecomment-247767027