How can I display the source text of an iPad email?

I think that is beyond the capabilities of Apple's stock Mail app on iPad or iPhone. Their Mail application on Macs provides that option, if you have a Mac.

Some webmail viewers provide source viewing of the headers. For example, the Mail component of iCloud.com provides an option to display complete headers. If the message in question isn't already in your iCloud email account, you can forward it there. but then you'll have to wade through additional layers of headers.

Full mail header information is difficult to extract using the iOS/iPadOS Mail App.

Notwithstanding any difficulties retrieving the Mail header, these are easily spoofed by the actual originator. If the header shows your own email address as the sender, the remaining header information is likely to be equally unreliable - and in all likelihood, the true source will be throughly masked.

Most emails of this type are designed to prey upon human insecurity - being intended to frighten the unwary into giving away sensitive information, or to fool you into doing something that you shouldn’t, usually to defraud you financially.

Do not respond to, or engage with, the message. Certainly do not, under any circumstances, attempt to contact anyone using the contact details that may have been provided within the message itself, or click any embedded links - as doing so simply confirms that your mailbox is monitored/active and will attract additional unwanted mail.

Simply delete the email from your mailbox - and then ensure it has also been similarly deleted from the Trash/Bin mailbox.

Anticipate receiving additional mail messages of the same or similar nature - each time deleting the unwelcome message.