Where is a display value of a referenced table in the mirror DB?
SnowMirror stores the data the same way as they are stored in the ServiceNow database. Not the way the data is displayed in the ServiceNow web application. The web user interface and even some exports (e.g. ODBC or exported XML) contain a display value of a referenced record (e.g. incident caller name). However SnowMirror downloads only a sys_id into the caller_id field. And again to obtain all the data you need please download two tables (incident and sys_user) and join them together.
The same applies for journal fields such as incident comments. These records are stored in the sys_journal_field table and the have to be joined with the incident table.