In my opinion, for cascode current mirror, the accuracy of output current is dependent on the matching of the under two transistors. if the under two transistors have the same Vds, the output current is the same with the input current, even though the transistor operates in linear region.
for cascode current mirror, the dynamic output resistance is very large and the output resistance of the under transistor is small compared with the whole dynamic output resistance of the current mirror. the result is that the change of the output voltage of the current mirror has little effect to the under transistor. this phenomenon is just like the voltage divider.