
Mangas that depict a relationship between an adult and a teenager are usually problematic because of huge power imbalances—the high school students have less experience, less resources, and less social capital so they can be easily manipulated by the older person into doing things they don't want to. These series often feature sexual assault :-(
On the other hand
although I would NOT advocate a relationship between Hayame and Issei IRL, i'm more okay with the idea of a relationship between them than I'd usually be.
Issei isn't in danger of being manipulated or taken advantage of by Hayame, because he holds a lot of power. He's the wealthy grandson of an inn owner (her employer) who houses and feeds her. She is a homeless divorcee without a high school diploma. Hayame is a good person who's not going to force him into stuff. Even if she wanted to, she wouldn't have the power necessary to do it.
age gap relationships are bad because they are dangerous for the younger partner, since Issei isn't danger, im a lot more willing to overlook it.
why didn't she tell him that she made the cake though?? >:0