Even a very large goblet is still going to be smaller than a breastplate or such for armor ... No matter how thin. And we have no clue how much cuendillar would weigh. Even if it is incredibly light it will still hinder mobility compared to leather etc as it does not have flexibility. No matter how you look at it armor reduces mobility, and that can be fatal in a melee ... As others have pointed out there are still gaps in even the best armor and realistically no armor will protect against channelers. Maybe a nice ceremonial suit of armor for the King or the Amyrlins Warder would be cool, no matter how you spin it the advantages just aren't really there.

If you want "out of the box" and you are going to use cuendillar as a defense I would think it would be smarter to coat a cities walls with it or something along those lines ... Having literally unbreakable walls or gates would be far more beneficial than a few suits of armor that may or may not give a handful of warriors an advantage against a Trolloc.