Burnett doesn't pull her punches with Ermengarde: she has "light, rather dull, blue eyes;" she's a "fat child who did not look as if she were in the least clever;" and, when we meet her, she's gnawing on a hair ribbon while forgetting basic French words.
Yeah. Who wouldn't want to be her best friend?
But Sara is our heroine for a reason. When all the other girls laugh at Ermengarde, Sara doesn't join in—she gets mad: "it made her feel savage when she heard the titters and saw the poor, stupid, distressed child's face" (3.6-7).
And because Sara is willing to look past appearances, she finds out that, beneath the not-too-attractive exterior, Ermengarde is a loyal and true friend. When Sara becomes very poor and all the others start ignoring her, Ermengarde goes out of her way to climb up to the attic and find out why Sara has been cold and distant. She basically begs to stay friends: "May I creep up here at night, whenever it is safe, and hear the things you have made up in the day? It will seem as if we were more 'best friends' than ever."
So in the end, we just don't care that Ermengarde isn't pretty and that she can't speak French. She's a good friend—and that's what really matters.