Generally speaking, most people grieve the loss of a parent. In fact, for many people it is the biggest loss of their lives.
In your situation, you did not know your father very well. If I could interview you in person I would want to know more about your relationship (or lack thereof). It would be difficult to grieve someone that you did not know well or who was not a big part of your life. That would be true even if it were one of your parents.
Another possibility is the one that your mother mentioned. It’s the idea that you are “numb” to your father’s loss. That is realistically possible. It is possible that your numbness is a psychological defense mechanism. Psychological defense mechanisms are not within your control. It may be that you are not psychologically ready or able to handle the loss of your father, at this point in your life. You may be better prepared to handle his loss later in life. If so, your numbness will disappear and feelings of sadness may emerge.
Unfortunately, I do not have enough information about the relationship between you and your father to know if your lack of grieving is appropriate. If this is something that you wish to explore more thoroughly, you should consider seeing a therapist. A therapist could gather many more details about your situation and better assess your lack of grieving. I wish you the best. Please take care.