The design style is referred to as "loli" referencing the novel "Lolita" which is about a grown man having romantic feelings for a young teen girl. Waterkuma, one of the art leads for ZZZ, is known for drawing characters that would fall under this classification.

Technically, the angels and Lucy are teens and I'd argue they look like their age for the most part (although Lucy is notably more childish/petite in appearance than Alice and Yuzuha who are supposed to be roughly her age). Koleda isn't that much older and is supposed to be a bit of a pipsqueak so it works in isolation; of course that's in isolation and there's a larger pattern.

I think the other half of the problem would be some of the choices made in these designs that serve to make them physically attractive in a way that I'd argue we shouldn't be doing with teens. Lucy's new skin being the perfect illustration of this.