One of the topics that Steven Universe handles better than most of the other shows on television is its depiction of loss and grief. The Crystal Gems and Greg have all lost Rose, and they are all shown to be coping with that from time to time. Steven has to cope with never having known his mother in the first place.
Polygon featured a great article on this, taking the point of view of two of its staffers who are coping with the recent loss of their mothers and what they felt while watching Steven Universe bring it up. Check it out!