Hi, regarding your concerns about your Mexican marriage certificate as part of your application:
Once you determined your application package by entering what country your spouse being sponsored resides (Cuba), and select Mexico as the country that you need to submit documents, the results will tell you specifically the country specific requirements for Mexico official documents, such as your marriage certificate (please see below) [copied/pasted online search]:
Country specific requirements
Cuba - Additional forms for residents
- There are no extra forms for this country.
Cuba – Instruction(s) for documents
- If you hold a passport from this country, you and your family members included in the application must provide:
- Your/their “solteria”, if unmarried.
- If you are providing documents issued in/by Cuba, provide originals of all documents required by the document checklist. The documents will be returned to you after a decision has been made on your application. Reminder: If your documents are not in English or French, provide a notarized (certified) translation with a copy of the originals.
Mexico – Instruction(s) for documents
- Official documents issued by this country must be government-certified true copies (copies certified as authentic by the issuing governmental authority, signed and sealed).
https://www.canada.ca/en/immigration-refugees-citizenship/services/application/application-forms-guides/guide-5525-basic-guide-sponsor-your-spouse-partner-child.html
I am not sponsoring a Cuban spouse, nor do I have Mexican documents to submit, but I believe that if you did follow the above instructions as per CIC, your application should be okay.
Best to you.