"Mim-mouthed" means "affectedly prim or proper in speech."

She was not wanted in that painfully prim cottage, and had been made to understand it from the first day.

probably &c adj.; belike^; in all probability, in all likelihood; very likely, most likely; like enough; odds on, odds in favor, ten to one &c; apparently, seemingly, according to every reasonable expectation; prim=a facie [Lat.]; to all appearance &c (to the eye) 448.

And into this uncomfortably prim and proper household little Gabrielle had suddenly been introduced.