Mama Mousekewitz is the mother of Fievel Mousekewitz, Yasha Mousekewitz and Tanya Mousekewitz. Mama is not a very prominent character in the series, but from what we see she balances Papa's carefree style of parenting with a more strict, disciplined style of her own. She is almost always taking care of Yasha, the youngest of their children. She also does much of the cooking in the Mousekewitz household. Taking care of a family, especially with Fievel running around and getting himself into trouble all the time, is quite the job. But somehow she manages. She is a lot more pessimistic and realistic than Papa.
In the first movie she calls his tall tales about America nonsense and is quick to rub it in when they discover there really are cats in America, and in the second movie, she warns Tanya, after catching her admiring a group of actors, not to stare at people less fortunate than herself. But without her the Mousekewitz family would likely fall into chaos, she's the one who keeps things running. If not for her Fievel would never take a bath, and we can't have a filthy little boy as the main character of the series, now can we? It seems likely that her first name is Emily. The mouse at the Castle Garden immigration center mentions it briefly in the first movie when registering the Mousekewitz family. This is only a fan theory, though, he could be saying "family", or Emily could be Yasha's American name.