My tiny house is 144 sq ft, average ceiling height 9 ft. lets call it 1300 cubic ft of air. That is being generous and not subtracting space of furniture cabinets etc etc. 1 foot of 4 inch pipe has a volume of about 150 cubic inches. I have about 100 feet of pipe, giving me 15000 cubic inches of underground air. Convert that back to cubic feet gets me around 8.5 cubic feet of air. That 8.5 is just a drop in the bucket when mixed in to the 1300 cubic ft of air in the tiny house. And unfortunately I haven't even factored in the time the air needs to stay underground to have any chance of cooling let alone dehumidifying.
As I stated before math is NOT my thing but feel free to check my work and play with the calculators yourself.
If you come up with an idea for a under ground radiator that could multiply the effect please share!