It’s likely a Matercard requirement to show the 
mark because ‘Mastercard’ will be a registered Trade Mark/Trademark in multiple territories. But there may not be a requirement on sizing, so Monzo has chosen to print it (therefore being compliant) as small as possible (therefore doing less visual damage to the look of the Monzo card while remaining compliant)
Edit: It was also discussed when Plus v1 launched in 2019: