There's nothing wrong with this movie, if you take it for what it really is: a romantic comedy with musical interludes and two-dimensional characters. If you expect it to somehow reflect the characters in the famous comic book series of the same name, you'll be very disappointed, however.
Pretty much everything from the comic book is changed here. In the comic book series Bécassine is a kindly young Breton living in Paris as the governess for Loulotte, the niece of the Marquise de Grand-Air. She is naïve, certainly, and not familiar with many of the ways of the world, but she is very even-tempered, devoted to her mistress, and not clever in either a positive or a negative way. In this movie, Bécassine is played by the same actress who just the year before had played the Parisian maid in Les Règles du jeu, the quintessential French movie about, among other things, the superficiality of the aristocracy. Dubost carries too much of that character over into Bécassine, her next role. She turns her into a sometimes nastily clever trickster who, like the maid in Les Règles, is constantly making fun of the aristocracy. A completely different character.
La marquise de Grand-Air, her mistress, is also different. In the comic book series, she is a very kind, and remarkably egalitarian woman. In this movie, she is just the same old stereotype of the snobbish elderly aristocrat.
The niece, Loulotte, is replaced with a daughter, Anne, who is old enough to be the female romantic interest here.
The plot - a group of thieves disguised as aristocrats to prey on unsuspecting hosts - is an old one, though not altogether foreign to the comic book series. In Bécassine aux bains de mer, for example, there is a French couple who disguise themselves as Spaniards to sell pottery left to them by an eccentric relative. They aren't really thieves, but the idea is not altogether dissimilar.
There are a few lame jokes aimed at poking fun at Bretons from a small town. Bécassine has a pet pig - think a less intelligent Arnold in Green Acres - and her uncle, Corentin, is a drunk. Not much is done with either of those threads.
There are a few pleasant musical numbers that have nothing to do with the rest of the movie.
In short, a pleasant but forgettable movie.
