I can think of three reasons to obscure or hide a character's face in a story.
- Hazy memory. Another character's memory of this person is unclear, so their face is not shown. This has actually been done in Nisekoi and is actually a major driving point of the plot, since Raku Ichijo can't remember the face of the girl he made the promise to. In flashbacks, the girl is depicted without showing her face.
- Focus. This character is not the current focus of the story, so their face is not shown. I saw a good example of this in the Watamote anime, at the end of episode 8. When Kii-Chan is going home, she is in the car with her mother, but her mother's face is not shown. The focus is on Kii-Chan sharing her experiences for the day, and her mother is just there so she has someone to talk to. Her mother is not the focus of the story.
- Veneration. The character is too important to show, or showing them might somehow reduce their mystique. For historical or religious figures, one might not want to show their face in order to glorify them. For instance, figures like Saint Anselm, Oda Nobunaga, and the Buddha might have their faces obscured to show that they are too great to be depicted.
I suspect the case of Ichijo's mother is a combination of all three.
- As previously mentioned, Nisekoi made extensive use of hazy memory in it's story. It's unclear what exactly Ichijo's relationship with his mother is like, but given she doesn't appear in most of the story, it's possible Ichijo hasn't seen her in a long time, and thus his memory of her is hazy. Likewise, the other characters haven't seen her in a long time, if at all, and thus her face is not depicted in flashbacks.
- The focus on the scene where she returns may be more on the information Chitoge is getting from Raku's mother rather than his mother herself. The character is less important than the revelations she's sharing and Chitoge's reactions to these revelations.
- The veneration point, I think, is the most important one here. Ichijo's mother is the author of the Zawze in Love children's novel, which is so important throughout the series. This book has almost mythic importance to the children, and they act out the events in the book through the keys and locket they have. As the author of this book, Raku's mother is a highly important person. Depicting her may lead to disappointment, as she would lose some of her mystique, and that in turn could make the children's book lose mystique. In order to maintain the mystique around her, and thus preserve the mystique of the Zawze in Love story, the author decided not to depict her appearance.
As mentioned in the other answer, a character's face might also not be shown due to constraints. Designing a character takes some time and effort. Shortcuts might be made for less important characters and background characters to get the manga out on time. I personally don't think this is the case for Ichijo's mother, but it is a point worth considering.