Should be a penis
It’s not a door knob joke. It’s a penis joke. British humor.
Well, considering the mug in question has a literal doorknob for a handle... probably, yes.
No, in UK we call people a knob or knobhead like dick head, it's just a shock factor mug so you read it and be like, ahhh doorknob.
That mug would be good for scooping or pouring, but crao for actually drinking out of.