This fictional character derives from a real person, Erna Nissen.
Typical jokes, usually narrated in heavy Missingsch dialect, tell of ineptness, bad luck, naivety, and low culture.
When she was to christen a ship to be named Little Erna, the champagne bottle didn't break.
When her family moved to Hamburg, her brothers started telling this anecdote, as well as other jokes of this type.
[1] Writer Vera Möller (1911-1998) collected many Little Erna jokes.