how does the plight of the seamstress illustrate one of the main flaws of the revolution?

One of the main flaws of the revolutionaries was their overwhelming desire for vengeance above all else. The seamstress (I'm thinking you're talking about the knitting of Madame Defarge here) is a symbol of how death was wrought at the hands of the peasants.

I take it back, you're referring to the seamstress who was unjustly sentenced to death. This was one of the Revolution's greatest flaws indeed. So many people were killed for absolutely nothing due to the mass hysteria and bloodlust of the revolutionaries. Innocents were slaughtered in the name of revolution and liberty; they murdered many of the people they claimed to be fighting for.