Lambda is a limitation to PSRR in simple circuits.
Adding headroom increases all drain currents in
current mirrors, gain stages.
Cascodes produce a quasi-constant headroom
on key current source / mirror devices.
Cascodes are not a panacea. Sometimes the
node in question may be sensitive to the
cascode device's capacitance (to where?) and
really wants "pinned" to the other rail somehow.
For example if you are driving the PMOS pass
FET with a cascode NMOS its Cdb (in vanilla
CMOS) is ground referred and supply activity
will push HF "stimulus" into the gate. The
same is true of a simple mirror, but the cascode
probably has flattened the I-V characteristic
and made the gate node very high impedance,
pulling the pass FET pole to lower frequency
(gate drive less effective @ HF, so the error amp
becomes ineffective as a HF PSRR actor).