Measuring the resistance of USB car chargers
Is measuring the resistance of the circuit in this way useful?

No. A look at the sample schematic below shows you that the only return path between your meter probes is through IC1. This will have a complex circuit of transistors, etc., and most will require more voltage than your test meter can supply before the circuit can turn on.

Figure 1. A 12 V USB power-supply. Source: dmohankumar.

Presumably, there is a switching DC-DC converter inside the charger, and on the average, the charger has to consume a bit more power than the power given through the USB ports.

Correct, although technically it is not a charger. It is a 5 V power supply. The charging is controlled inside the phone and it will regulate the current as required to safely charge the battery.