The Event Guest List report is an older legacy report type. It was created at a time when we did not offer the ability to add most constituent profile fields during event registration. Since the report was created before we had the ability to add mobile phone numbers, the column is not included in this specific report. Here's our documentation containing the Event Guest List reporting columns. The Mobile Phone column is not listed here.

You can get the mobile numbers by running an Event Attendees report in Report Writer.