Right, if an animal passes the test that's a strong indication of self awareness, but if it doesn't then consciousness cannot be ruled out. I would imagine most animals are self aware at least on some level because being able to model oneself within your environment is a useful property. Any organism with a relatively sophisticated internal model would naturally have itself as part of its model.

The mirror test is certainly somewhat debatable in determining the degree of self-awareness and intelligence. For this reason, today it is determined not only with this, but with a set of tests that includes the general ability to solve problems to determine the intellectual capacity of an animal. It is always the question if the animal does not correspond to the mirror test, if it is due to lack of understanding, lack of interest or other reasons, little childs also don't pass the mirror test. It is to find out if the animal knows exactly what it is doing.