I didn't think it was all that bad.
Rarity's been played up as prissy and fussy throughout the entire series, so it makes sense that she'd turn that to her advantage like that. Had the captive been Fluttershy, for example, the storyline might have had her getting the Diamond Dogs to feel sorry for her. Pinkie Pie would have annoyed them to death with constant babbling. Of course, Applejack and Rainbow Dash would have probably just fought their way out. The true "Tom Sawyer" tactic sounds more like something Twilight would do.