Okay, I understand why she has: Mismagius (Magician) Absol (Black and White) Jolteon (Master Spark) but why Dusknoir, Garchomp, and Pigeot? If they were gonna give her a flying type, I think Staraptor'd be a better choice for her, and I can't think of how the other two match her theme.
Pidgeot's entry goes on about being able to create whirlwinds with only a few flaps of its wings, fly at Mach 2 speeds, and its "dazzling plumage". So there might be a connection with Marisa's spellcards being a little "shock and awe" and how she's constantly training to get stronger.
Pidgeot is also known for its insane speed. Only Aya and Youmu exceed Marisa's agility. I'd wager the speed and flight is why she picked it, since it can keep up with her broom.
And Garchomp probably is the team's mega stone holder. I don't recall any of the others having a mega form.
Even though Pidgeot is famed for its speed, it's not a very fast Pokemon in the games. In this team, it's actually outsped by half the team (Garchomp, Mismagius, and Jolteon being faster). Mega-Pidgeot, on the other hand, is only outsped by Jolteon.
So it's a legit tossup who has the mega stone? That's actually interesting.
Plus knowing Marisa, she probably figured out how to channel power into multiple mega stones at once. What's worse than Mega Garchomp? Mega Garchomp teamed up with Mega Pidgeot and Mega Absol.