This is a common trope in many stories, where a character's mother is either absent or deceased. It allows the story to focus on the relationship between the father and child, and often creates a sense of loss or longing.
However, in the Broadway musical adaptation of Aladdin, there's a slight difference. While still not seen, Aladdin's mother is briefly mentioned in a song ("One Jump Ahead"). In the song, Aladdin describes how his father used to tell stories about her and how she was "the best thing" to ever happen to him. This implies that she was present in Aladdin's life, but passed away before the story takes place.
So, while the movie doesn't offer any explanation for Aladdin's mother, the Broadway musical suggests that she was a significant figure in his life but is no longer alive.