Why cascode transistors contribute little to overall noise in an OTA?
Noise contribution of a device at the output will be multiplied by the gain from the noise source input to the output. Voltage gain for a transistor can be approximated as $$A_v = \frac{Z_d}{Z_s},$$ where \$Z_d\$ and \$Z_s\$ are impedances seen at drain and source respectively. For a cascode device \$Z_s\$ is high since it sees the output impedance of a transistor. So its gain is very small, consequently its noise contribution will be negligible compared to the other devices which will have high gain multiplied by the noise sources.