As revealed in "Glocken Around The Clock", when she gets fussy, she does a terrible thing - specifically, she makes a sad face. Things leading up to the Terrible Thing are when she wiggles her ears and her hair curls.
She can be remarkably destructive, and is capable of breaking things no baby her age should be able to break.
Mathilda looks like her older brother and parents, but she is smaller, and has a pink pacifier in her mouth, as well as a strand of hair with a red bow on it. Unlike Leo (and other characters), she has no visible toes.
Her name is a reference to the Australian song, "Waltzing Matilda".