Palinkasocsi“Which is better to use: the singular or the plural?”
I don't think it's wise to invent a new constituent for the sentence on the basis of your imaginings, so I don't believe the noun
mimicry has anything to do with it.
There are only two possible antecedents of
that:
indices (plural) and
quotation (singular). So it's a matter of what the relative clause is about.
Is it talking about indices that are even mimed by hand gesture?
Or is it talking about quotation that is even mimed by hand gesture?
Personally, I'm drawing a total blank on what either of these things might mean.

CJ