Input filter capacitor ESR is a big deal in low voltage
POL buck converters. The capacitor sees current
equal to IOUT - charging during output switch "off"
interval and discharging during "on" interval. So every
electron you throw to the output gets dragged across
the input filter cap ESR, twice. IIN of the DC-DC power
stage is a square wave current, and in-the-moment
on high fSW converters, little of it comes from the
power source, most of it from the close-in decoupling.
The less input voltage ripple you see, the more true
this is.
Output filter only sees the output inductor ripple current
so its ESR matters a lot less to efficiency. ESR and ESL
are however both key to minimize load-step deflection
in the interval between application of the new load, and
the control loop coming around.
At 2mA however I'd bet it's all about switching losses
and not so much either filter.