I get a pop-up message asking to create a mobile account and would like to disable it.
You have created a Task Sequence with Join Domain step and have checked the Create mobile accounts at login check box.
To disable the pop-up message please add a new Install Software step in the Task Sequence, right after the Join Domain step.
It should be an empty package with the following command line:
/usr/sbin/dsconfigad -mobileconfirm disable