Mr. Mumble.
"The Great Gatsby": on a party,
A tray of cocktails floated at us through the twilight, and we sat down at a table with the two girls in yellow and three men, each one introduced to us as Mr. Mumble.
question: It's obvious "Mr.Mumble" is not their real name, but what does it mean here?
thank you for your help and your time

Question

Mr. Mumble.
"The Great Gatsby": on a party,
A tray of cocktails floated at us through the twilight, and we sat down at a table with the two girls in yellow and three men, each one introduced to us as Mr. Mumble.
question: It's obvious "Mr.Mumble" is not their real name, but what does it mean here?
thank you for your help and your time

Answer

The speaker is making a joke. Or at least is being mildly facetious.

Instead of saying "I was introduced to three men, but the introductions sounded like a mumble to me and I couldn't understand them," he says "they were introduced to me as 'Mr. Mumble.'" Part of the point is that this is a familiar problem in parties, He expects us, the audience, to recognize a familiar situation. We understand that it is impossible to misinterpret a mumbled name as the actual word "mumble," to say nothing of then name "Mumble," but we get the point.

Answer

It means that the narrator didn't hear what the men's names were, and probably he didn't care what they were.

Não perca a oportunidade de aprender uma língua no conforto da sua casa. Navegue pela nossa seleção de professores de línguas experientes e inscreva-se na sua primeira aula agora mesmo!