Currently when trying to add Mondo card as contactless card into TfL account for tracking journey history and charges (which are not really transparent and sometimes baffling) - TfL site will fail with following error:
“Sorry, your card could not be added. Please ensure the card details you provide are correct and the address you enter is exactly as it appears on your payment card statement. If you continue to experience this message please contact your card issuer as there may be a problem with your card.”
Maybe they just don’t support prepaid cards?