Nethercap is a mushroom... and all you did was essentially make a mushy concrete.
I don't see why it would essentially make super stone armor especially when concrete is significantly weaker then the stone it originates from.
I am being harsh but I am against "making magic for magic's sake" which this is mostly "well it is both rare and deep underground... of course it is much better!".
I mean Adamantine was a super material, but its purpose is directly tied into the lore.
1. I'm aware nethercap is a mushroom, but it acts like wood, and thus it should act similarly when you get down to pulping it. The fixed temperature would keep the water frozen, as pykrete has a melting temperature of ~45*F, well above the fixed temperature of nethercap. So why it would be "mushy" makes no sense. Just because it's called something-krete does not mean it's based on concrete, pykrete was named as it was on account of it fracturing about the same as concrete despite it being mostly ice.
2. It doesn't make "super stone armor" It makes higher-tensile strength (as in it can bend a bit more than most) moldable stone... which is still crap compared to metal or higher strength stone like Marble and Granite if they were modeled for their full material properties. (sadly "stone template" is just a basis. Stone varies widely in strength obviously)
3. I'm not trying to make magic or anything. I'm trying to take an already present magical object (below freezing point of water woody mushroom.) and utilize it in a way that makes sense given real life physical interactions with an object of the given temperature and other outstanding known material properties - in this case, making pykrete out of it.