Who is the German character Mouse Marie?

There is no such character, but you may be referring to the story of 'the mouse of the Marienkirche' (the mouse of the Maria Church) in Lübeck, Germany. It is depicted in the church choir to remind the Lübeckers that small causes may have big consequences.

The saga goes that Lübeck was under siege from the Danes, but its citizens had sworn to defend the city as long as the big rosebush next to the Maria Church would stand and bloom. But one day the rosebush had wilted and died; it turned out that a mouse had made a nest among its roots and the mice had gnawed through the roots, causing the bush to die. Soon after the Lübeckers had to surrender their city to the Danes.