A birth certificate document (legal) asks for mom's maiden name; mom was adopted and never married, and her last name has changed. Which last name is her maiden name? The name before or after the change?
A maiden name is "the name or surname of a woman before taking her husband's surname upon marriage." (OED Online)
In the case of a woman who has never married, any hypothetical marriage and taking of a husband's name would have to be in the future, so the name she legally has now is her "maiden" name.
However, this question is often asked as an informal identity-check, as a fact that only you (and presumably your other family members) are likely to know. For example, it may be used as a password-recovery question, or as a way to identify yourself over the phone when making inquiries about your credit card. In that case, your mother's birth name would presumably be less a part of public record, and a more secure option (that children of unmarried, non-adoptees don't have). Just don't forget that that's the choice you made!
The name after the adoption should be used.
The purpose of including a maiden name on a birth certificate is to allow someone who wants to track the next of kin of a child for genealogy or inheritance purposes to link the mother's name in the child's birth certificate to the birth certificate and marriage certificate (if any) of the mother. (The same question is asked about the person's mother's maiden name on their death certificate.)
Upon adoption, the name of the mother on the birth certificate would have been changed.
So, if the original name of the mother were used, a records search for the mother based upon the name found in the birth certificate wouldn't locate the mother's birth certificate or other vital statistics records (for example, the birth certificates of maternal siblings of the child for whom the birth certificate is prepared).