A marriage certificate is not required for your mother. It is sometimes required for father. If you are sponsoring your mother, then you need your birth certificate (with her name on it) to prove the relation and her birth certificate or alternate documents.
This is an extract from the US consulate website (some information is specific to India):
Proof of Relationship with the Petitioner
For Children (IR-2, IR-3, F-1, F-24, F-3 and K-4 Visas) and Parents (IR-5 visas) of U. S. Citizens, Children of Family-Based Visa Applicants (F-3 and F-4 Visas), and Children of Fiance(e) Visa Applicants (K-2 Visas):
Primary: Old Indian passports, birth registrations issued within one year of birth
Secondary: Secondary school leaving certificates, ration cards, baptismal certificates, family documents such as land transfers and wills, original wedding invitations, college degree certificates, mark sheets, property certificates, family photos or other reliable documents that show the relationship.