。
She's an easy mark for flattery.
Literal
She [topic-は] easily flattery [in-に] easy-to-be-taken-in.
お世辞に乗る (literally 'ride on flattery') is the idiomatic phrase for falling for / being taken in by sweet talk; the お is the polite/honorific prefix that's fused into 世辞 (originally 'small talk / pleasantries'). 乗る in this metaphorical sense covers being pulled along, going along with, or falling for — same as in 話に乗る ('go along with the conversation') or 誘いに乗る ('take up an invitation'). The ~やすい ('easy to / prone to') turns the verb stem 乗り into a personality-describing adjective. The whole sentence reads as a personality assessment: she's vulnerable to flattery.