I wonder whether poupelet is a diminutive. 'Poupel' seems to be another surname, annoyingly.
Anyway, here's the OED's suggested etymology for poplolly: 'App. alteration (perhaps after LOLLIPOP n.) of either POPELOT n. or French populo baby, tot (c1490 in Middle French; < poupelin (see POPELOT n.), with expressive alteration of the ending (compare -O suffix)). Compare earlier POPELOT n., POPLET n.'
Checking the etymology for popelot gives: 'Either < Middle French popelin baby, tot (c1225 in Old French; French poupelin; < an unattested post-classical Latin *puppa (see POPPET n.) + Old French -elin < -el -EL suffix2 + -in -INE suffix4), with suffix substitution (see -OT suffix), or directly < Middle French poupelot baby (although this is first attested much later: 1564 as pepelot; French poupelot; alteration (with suffix substitution: compare -ot -OT suffix) of poupelin). In later use probably < French poupelot. Compare later POPLET n.'
Poupelot is only one letter away from poupelet, so I wonder whether they're related.
According to Dictionary.com, poplolly is an affectionate term, meaning something to the effect of "little darling." Their editors suggest it comes from the French word "poupelet," which I've been unable to find any further information on. I tried the OED, Etymonline, and the Babelfish translator. Googling only seems to bring up information about Poupelet as a surname.
I also found the Dictionary.com definition to be at odds with the definition from the OED, which defines a poplolly as a mistress. The latter definition seems to fit with the usage in the novel I'm reading, which is Suzanne Enoch's After the Kiss (p. 250).