In this case I think the context demands that we take talking heads as a participle clause modifying Mama—many linguists (e.g. John Lawler) would probably say it is a relative clause reduced by whiz deletion, who is talking. The purpose of the head clause this is Mama talking is clearly to identify the speaker, which might otherwise be ambiguous.
In any case I think we have to rule out talking as a gerund. We use talking that way only to designate the activity, not the content.
Mama's talking while he was trying to read annoyed Papa.
If the content of Mama's speech is intended we have to incorporate it by allusion.
Mama's talking this way annoyed Papa.
