It is not safe to use diesel fuel in a kerosene heater. According to the National Kerosene Heater Association, the American Petroleum Institute and the Consumer Product Safety Commission, only K-1 grade kerosene should be used.
Diesel fuel is not flammable in its resting liquid state. Unlike gasoline, if a lit match is thrown into a bucket of diesel fuel, it does not catch fire. This is because diesel fuel must be under very high physical pressure to ignite. For this reason, it is unlikely that diesel fuel would work at all in a kerosene heater, even if the safety concerns were not a factor.