It is. Printing the TM is a Mastercard condition of use, as I understand it.
Understandly Monzo have chosen to print it as small as they possibly can, so it doesn’t look too obvious and out-of-place. This means they’re reaching the minimum possible printing resolution, which is why it’s hard to read.
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: