It's because it's the "anime mum hairstyle"; most of those mums belong to a shonen/young protag and shonen or other young protags traditionally have dead parents. Ed & Al's parents: Dead. Eren & Mikasa's parents: Dead. Harry Potter's parents: Dead. Spider-Man's parents: Dead.

Why do so many MCs have dead mums? My dad who was an English Lit teacher said it's because parents tie teenagers to home IRL, so it's much harder to have a protag who goes around doing cool stuff while his mum keeps telling him he can't.

Because she is not a mom. If she becomes one and keeps the hairstyle she dies, but as it is now she will live, even if it is Overlord and she is a human.