Props to /u/Eudicot in reddit explaining it better than mine:

Yui (Tracer) is the main character of the series and is a quirky derp overload. She plays the guitar and is one of the lead singers of the band, so Tracer fits as she's sorta the front of the band and series while also being quite quirky.

Azusa (Widow) is the tryhard of the group, and becomes somewhat upset often since the rest of the group doesn't practice. I guess the Widow is meant to show that she is the tryhard while also fits as she sorta ends up as somewhat of a foil for Yui.

Mugi (D.Va) is super rich but just wants to experience the normal pleb life. While she doesn't really fit D.Va, the idea that she'd play D.Va, and particularly that pose, makes since due to the fact that she'd be the one to do that pose despite not at all being that sorta person.

Mio (Mercy) is the shy one of the group and doesn't like to draw attention to herself. Mercy fits playstyle wise since she's more of a supporting role (bassist). Someone noted that she's tethered to Ritsu (Pharah) which is very apt considering that Mio is often bailing Ritsu out when she screws up.

Ritsu (Pharah) is the brash one. She's loud, obnoxious, and the "president" of the band. She's the one who usually suggests what to do and somewhat commands the band on what to do.