I pretty much have to argree with ordjen.
The fact the underside of the floor is exposed to the open air it should be a priority of insulating it fully and properly. Batt insulation ( unfaced ) fully filling the joist bays with rigid foam apllied to the bottom side of the joists. The rigid foam will provide a continious thermal break greatly improving the insulation performance.
You can apply OSB or plywood over top to prevent critters and / or insects chewing the insulation.
Considering the area under the floor is basically open to the outside then the plastic on the ground isn't doing much of anything --- other than keeping the weeds down.