Pedanticaly, No they are not because they have never been a stock part on any car produced for the Japanese domestic market.
But obviously as being JDM has evolved to mean any part or modifiaction that has been inspired by what goes on in japanese car culture then yes.
They are often found in a variety of japanese scenes, from boso and drifters through to VIP.
On a related note, anybody know what the thread on a Micra gear leaver is, its definately not a standard metric coarse, I've roughly measured it to be M10x1mm. I've got a scheme for a pretty unique custom gear knob, but I need to sort myself out with a tap to suit the gear leaver shaft.