Sie ist eine süße Maus.

Is this a common idiom?

Both this and “Sie ist eine Süße.” seem to have been added by the same native German speaker on Tatoeba.

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I found it in a dictionary:
So eine süße Maus!
What a sweetie!

Italian Translation

È una ragazza carina.

another useless phrase !

It is not uncommon.
However, you have to be careful. It has never been the highest register to begin with, and especially nowadays it may come across as misogynistic, or at least macho.

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American lowlifes or gangsters (at least in some regions) sometimes refer to an available girl as a 'mouse."

I’ve never heard anyone refer to a girl as a “mouse”, but there are so very, very many slang expressions for such things that it would be impossible to list them all. In England, for example, they refer to girls as “birds”, which I always found particularly odd.