depends a bit of whether You regard a robe as uniformed habitus or not and what You mean by "uniform"
I shall guess that You mean clerical or military

So, whithout being a specialist in teutonic stereotypes, my guess would be that since german society used to have a draft towards military style uniforms (and maybe still has), an order of cultists might regard themselves as against the mainstream of their society and therefore favour robes or something similar that does not look militaristic.
Whatever it pleases You to do, squeezing in a monocle over a cowl might appear a bit overdressed
