My best guess is that this “Zero” means either looking/feeling horrible, like “nothing at all”, or that the number of guests in the ball had dropped down to zero.

I am not aware of any expression with the word “zéro” in French that would help here.

What was the native language of the writer? This might give clues, by looking at how “zero” could be interpreted in their language.

I guess the intent was:

On ne ressemblait plus à rien.

Which is a common way to say:

We looked terrible.

Literally it means “we looked like nothing”, with the idea that we looked so terrible it it impossible to find something to compare with. Here that could be illustrating the fact the people are very tired and worried.

Anyway it is pretty difficult to be sure about that. It could also simply be nonsense.

It sounds like it might mean “We danced only one dance after dinner, and at such a time [e.g., so late] that it was more like zero.”

My thinking is that “on” might actually be “où”. It might be “où c’était plus ressemblant à un zéro,” although a native French-speaker would be unlikely to use that wording.

After quite a long wait, I have just received a picture of the original letter from the author of the article it is quoted in.

No wonder it didn’t make any sense—half a sentence had been left out! There are two était’s almost right above each other, at the same place on the line, and the transcriber had simply skipped the whole line between them.

This is what the letter turned out to actually say:

… & à une heure on s’était retiré, à l’exception de la Stricker qui resta à garder le Héro qui était plus ressemblant à un Zéro.

This, obviously, makes an awful lot more sense. It even makes it clear what the ‘zero’ is all about: it’s supposed to rhyme with ‘hero’!

Thanks to all for their creative thinking and attempts to make sense of what eventually turned out to be a phantom phrase.