Dreaming of your deceased mother signifies that you miss them, and something in the past is missing closure. Something in the waking hours could trigger the memories of the death of your mother and cause past issues to resurface.
The death of a loved one can be upsetting, regardless of whether they are close to you or not. In some cases, it is even worse if you were close to them and had an emotional bond like with a mother.
This all means that it is important to remember your loved ones no matter how much time has passed before they leave us. Take some time off to visit their cemetery and pay your respects.