Do I need a certificate from the Malaysia Embassy to get a second 30-day tourist visa extension in Thailand?

There is no "second 30 day tourist extend stamp" :(

IF you bought a tourist visa from a thai consulate before you came here and were stamped in for 60 days, you can extend that ONCE for 30 days at Chaengwattana Section J (which looks like you did already)

AND

If you need more time you get the letter from your consulate that says you can't travel due to covid-19 and go back to Section J (where you got your first 30 day extension) and apply for the emergency covid-19 extension.