No. You have both options.

Message in the file: add a Message column and write the text there, including variable names such as $CUSTOM1$. Every row can have completely different wording.

Message in the panel: leave the Message column out. Upload the file, choose the header that holds the mobile numbers in Select Mobile Number Column, then write the message in the message box and insert variables using the airplay icon inside it.

For routine campaigns where only the values change, keeping the message in the panel is cleaner. You edit the wording in one place instead of on every row.