In this case I think the context demands that we take *talking* heads as a participle clause modifying Mama&mdash;many linguists (e.g. [John Lawler](https://english.stackexchange.com/a/121619/24489)) would probably say it is a relative clause reduced by *whiz* deletion, <i><s>who is</s> talking</i>. 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.