"as dumb as they come". whats a good german translation for this phrase?

If the context is: "These people were..."

you have different options, depending on register:

low colloquial: ".... wirklich saublöd"

normal colloquial: "schrecklich/ entsetzlich dumm"

written: "von der dümmsten Sorte"

generally "so dumm, dummer (dümmer) geht's nicht"

You have to take into account that contest is all-important in translating.

so dumm wie die wirken

or plain german:

so dumm wie die aussehen

dum koff... litterally means dumb head. pronounced "doom caugh"