I briefly read the moral as something like this; something along the lines of "being exposed in the open was the worst thing the mouse could imagine, so it ran blindly away from it without asking what the alternatives were". I'm still not sure I actually get it.

Tangentially, keeping mouse traps in a house with a cat seems hazardous (though I could be underestimating cats). And I assume "day" and "chamber" are used abstractly.