I understand that there is a simple Round Edge Radius parameter in Principledshader that can be implemented, but how to implement MTXL material in Karma XPU?

The vop is called "Karma rounded edge", in Houdini 20.5.

You will find it when you are inside a Karma Material Builder.

The output of this goes into the "normal" input of the mtlxstandard_surface.

Perhaps this is what you are looking for?

https://www.sidefx.com/docs/houdini/nodes/vop/kma_roundededge.html [www.sidefx.com]

by default it doesn't appear in the tab menu in materialx builder only in karma material builder but it does seem to work in a materialx network with xpu if you paste it in.

Thank you both, this is working properly!