An attribute 'no_auto_map' exists out of the box that is meant to prevent automatic mapping of the field 'manager' in the sys_user table when transforming data to import users into the platform

Is it possible to  disabling this attribute 'no_auto_map' shown in the dictionary entry of the field sys_user.manager ?

Yes, this is possible to disable or remove attribute 'no_auto_map=true'

It was set up that way out of the box because, additional coding is required to fetch the manager value from LDAP and then attempt a lookup against an existing user in Servicenow

Out of the box, it is not possible to set the manger value from AD to the sys_user.manager because this is a reference field, so the underlying real value is a sys_id.

If the field was automatically mapped, then it would fail and throw a mapping exception.

It is possible to map but a little bit of additional coding is reequired to do the lookup from the name to return a Servicenow sys_id

The pre-requisite for the lookup to succeed is that the user manager name should already exist in Servicenow.