I don't know if you can import .mbox files directly into Airtable, but you could always import your mbox files into a Gmail account, and then use Make's Gmail automations & integrations to automatically bring in the emails from Gmail to Airtable.
Your Make scenario would be as simple as the screenshot below:
I think you can convert your mbox file to a CSV with Thunderbird and the ImportExportTools NG extension, then you can just import the CSV into Airtable directly