Depends. A lot of this depends on your age, your relationship with your mother, and the circumstances of the situation. If you are a child or teenager, then it would be good to briefly let you mom know that you are concerned and encourage her to seek help. If you are an adult, then you can tell you mom more clearly about your concerns and her need for help and support.
Love her & get help. Your mom may be feeling blue for many reasons: she could have a medical problem like depression, or she could be dealing with grown-up problems she doesn't want you to worry about. Tell her you love her, and that you are worried about her seeming so sad, and suggest she talk to her doctor, or another adult she trusts, like a religious leader. It is never a kid's job to fix a parent's troubles.