Burikko refers to a girl who likes to act all cutesy or innocent, usually adult women who fawn on men affectedly and act like little girls. Additionally, it can occasionally refer to a girl who fakes being cute, sweet, innocent and nice, but is actually a bitch. The use of it in this series refers to the more popular usage (it’s just that the girl is coincidentally, also a bitch).
(There is no satisfactory English equivalent, and I don’t believe in hamfisting things just to seem like a ‘better translator’. And obviously people have been doing this for ages, otherwise we wouldn’t have the word ‘sushi’ or ‘keiretsu’ or ‘ninja’ in English. They’d just be ‘vinegared rice rolls’ and ‘Japonic Cartels’ and ‘Sengoku mercenary agents’ or something, so you can get off your slightly large-ish pony.)