Should I say, "My mother's maiden name was Smith" or "is Smith?"
I think that one does not lose their maiden name. They may take the name of their spouse, but their maiden name remains intact. The maiden name of an individual is their name before marriage. They do not lose their name in marriage. They may choose to or have to adopt the name of their spouse. However, I do not see this as an automatic negation of their maiden name. Therefore, I think that the idea would be that one still holds their maiden name in marriage and it is not an immediate statement of the past.
With this in mind, I think that the statement becomes that "My mother's maiden name is Smith." In the sentence, the mother may have adopted the spouse's married name, but her maiden name is not an automatic denial. This would mean that the verb in the sentence should be present tense and not something in the past. The verb "is" would be used to reflect the present tense, indicating that something is still active and not in the past.
From a grammatical perspective, I think it is perfectly proper to use either "is" or "was." In most countries, a woman's maiden name generally follows her after marriage--as either a middle name or, in many countries, as the woman's legal name. So, in most cases, since a woman legally retains her maiden name along with her married name, the correct answer would be "My mother's maiden name is Smith." Grammatically, a change might be considered if the mother is deceased and was being referred to in the past tense; then, it would be proper to say "My mother's maiden name was Smith." Additionally, if a woman legally changes her name to no longer include her maiden name, it would probably be more accurate to say "My mother's maiden name was Smith."