I had to look this up. ''As crazy as a sh*thouse mouse'' means completely crazy. Nothing about the character of mice in general, ''mouse'' just rhymes with ''house''.
A sh*thouse is a crass word for an outdoor lavatory (uhm... fairly obviously). In Australia, when we describe something as ''sh*thouse'', we mean that it is completely useless. However, we use it in a longerexpression to mean something/someone is powerfully-buılt and solid: ''Built like a brick sh*thouse.''