According to Wanda, Maryann abused their magical powers. On the 28th of June 1914, she wished that Franz Ferdinand (the Crown Prince of the Austro-Hungarian Empire) and his wife would be assassinated, thus plunging the world into the First World War which led to the Second World War. It was for this reason that Cosmo and Wanda were taken away from her.
It is currently unknown how she was able to wish that the Archduke would be killed, because in the Da Rules, it states that a godchild cannot wish someone dead. However, it is possible that this rule was created because of her, or she found some sort of loop hole.
She was once wished out of Cosmo and Wanda's Hall of Infamy in their house by Timmy after he stole Cosmo's wand. She then tried to steal Wanda's wand and would have succeeded in getting her revenge, but was thwarted by Wanda, and put back into her painting where she belonged.
She is arguably one of the most evil godchildren Cosmo and Wanda ever had, as she actually had Archduke Ferdinand murdered through magic, and started the two World Wars, whereas in actual history, Archduke Ferdinand was assassinated by Gavrillo Princip. (noting in actual history, she may have wished for Gavrillo Princip to assassinate the duke and not fully getting killed on just solely magic.)
It is also worth mentioning that she also allowed Adolf Hitler to rise to power, meaning she has the highest (albeit indirect) body count in the series at 6 million (the amount of Holocaust victims).
The Hall of Infamy picture she was in was black and white and she retained her grayscale tone even after she was wished out of the picture.
Maryann is also very manipulative as she tricks Timmy into giving her Cosmo's wand to use it to gain revenge on her former godparents. This may also be the way she had Archduke Ferdinand assassinated as it is against Da Rules to kill someone. However, she may have manipulated events through magic and that led to the assassination. Either way, she clearly planned on doing this.
It is possible that she managed to obtain a Magic Muffin, making it possible for her to wish Archduke Ferdinand murdered without breaking Da Rules.
It is also possible that this wish was a secret wish and that she wished that Wanda or Cosmo forgot about it, like Timmy did in "Timmy's Secret Wish!"
A third possibility is that there may have been no rule in Da Rules concerning murder and/or assassination prior to June 28, 1914, and Maryann may have been the reason such a rule was made.
Maryann's appearance is similar to 1930's child star actress Shirley Temple. However her personality is very similar with Darla Dimple (she too has a similar appearance of Shirley Temple) whom is the main villain from Warner Bro's 1997 animated film Cats Don't Dance.
Maryann might've been inspired by another Shirley Temple look-alike Baby Doll whom first appeared in the episode Baby-Doll from 1993 series Batman: The Animated Series.