And Mantle has nothing to do with it
It was a wild guess because amd new driver release includes it... not that mantle support would actually works in minecraft, just that amd drivers are usually buggy
Minecraft runs on DirectX.
And no Java only supports Opengl, its why minecraft works on osx and linux.
If minecraft worked with DirectX, it would have better fps performance and have memory usage, as it would be most likely be using C++ instead of Java.