The cat sees not the mouse ever.

The cat does not notice the mouse. The saying presents a watcher who fails to see what it might be expected to see; watching alone is no guarantee of noticing.

The word “ever” leaves the extent of that failure open. The cat might never see the mouse, or it might miss the mouse on some occasions. Neither reading says why the mouse goes unseen.

Use the saying to remark on a failure to notice something one might be expected to see. It need not imply that every attempt to look will fail, and it does not establish fault on either side.