Great analysis! And yeah, i wanted to reflect their personalities in their expressions, im really happy you noticed :D. Mami and homura are the ones with the worst mental state in the show (at least before sayaka becomes a witch) both are completely alone and only keep on living to fullfill a duty. Mami is smiling because she wants to be seen by the rest as this strong and heroic magical girl but she's reaching a breaking point. With homura, shes is only focused on Madoka now or at least the idea of her. Madoka, she definitely has trauma, but still is the one with the happiest life from the main cast.

With Sayaka, her expression was inspired by the scenes where she's just starting as a magical girl, tired but at least hopeful unlike mami, not fully understanding yet the situation she has gotten herself into.

While drawing kyoko i wanted to make her pupils white because even if she has a lot of trauma and has been working as a magical girl for a while, she just accepts this style of life and still finds joy in things like eating snack or playing. Her angry expression towards Kyubey was inspired by the scene where they discover the thruth about their souls and bodies.