Small signal analysis shoulf be done but this is not where
analysis should end. LDO error amp and output stage may
well "wind up" under large signal conditions, at min or max
load, min or max line, and then small signal analysis may
entirely miss the operating point of interest.
PSRR of a PMOS LDO is often quite poor especially at high
frequencies. You have to choose compensation wisely,
some styles that are area efficient also act as "power
supply noise injection" paths into places where gain
will follow, and you're then going to see just what you
see.