Coming from a large fam, I do have relatives with step-moms/dads and for the most part the kids referring to aforementioned step parent is really up to them. No one forces them to call said parent mom or dad -- especially ones coming from a widowed parent situation.
Persephone, in the little time that we see her and Aeson, doesn't strike me as someone who would push Aeson to call her mother or yeah.
If anything, it seems entirely as Sammy pointed out his choice, especially when Rohie has pointed out that he lost his bio mom young and grew close Persephone to do so.
Imo, Aeson has that right to call her that and it's not up to the MC to question. Only time it seems apt is if MC seems to have a spat with them and wants to be rude and hurtful about it