Why give Ahri/MF SG skins when they don't fit the theme

They're not adult older, in this skin line. They're "we're seniors" older. I'm not sure about you, but bitches ruled cliques in my high school.

Syndra wasn't even originally part of team ahri, syndra's team all died and she was the only survivor. So she's all dark and fallen now because of it. The Star guardians just pushed her into team ahri because they had an opening. Ezreal is the new cocky kid on the block. Soraka is the voice of reason, the one nice girl who sits with the bitches either because they were old friends or because they want to be popular too.

Taric is less likely to get a star guardian skin because of his build. He's broad shouldered and heroic in a western sense, while riot explained they chose extra because his slender body type was reminiscent of boys in magical girl anime. That body type is common in eastern manga and anime, off which the line is based. Particularly star driver and Earth defense club.

Sona was more of the same. They said they wanted to highlight each individual in this set so they looked for different personalities. She'll probably get one next year

Annie barely had a personality at this point. They can't decide whether she is inherently malicious or not. Leading up to all kinds of head Canon.

MF's personality doesn't fit I agree, but that's only if we're looking at them as schoolgirls.

They aren't. If anything, they are senior star guardians (like if Lux and Jinx were freshmen, MF and Ahri are seniors) and Ahri expresses this through her experience inflated ego.

Ezreal is probably a new recruit to the vet team.

If they wanted to introduce SG that are new to the job, Taliyah's personality would fit. But it looks like they went for the more experienced route.

How many magical girl anime have you seen? They aren't all sunshine and rainbows, and a lot of the more recent ones are really dark and downright disturbing. MF and Ahri fit the disillusioned veteran role pretty well.