JDM is a japanese spec standard production car.
What kids refer to JDM is fender mirrors, super wide rims with skinny tyres stretched onto them, a car lowered to the weeds so it is useless on the road and defect material, etc. Really Bozo is the correct term. Google "Bozo style".
It is a bit like "granny spec", which really is an overdone version of what a granny would drive. ie, whitewalls, slammed, venetions, sunvisor, etc.
It is a LOOK, or a Style.