Mami is not a Witch!
Like I said, she's a Sorceress.
More seriously, you'd need to homebrew some mechanics to actually account for the way grief works for magical girls, and at that point you might as well go all the way and use some of the existing homebrew specifically meant to evoke magical girls. (I know
@AuraTwilight has a bunch, spread across a variety of D&D editions).
More interesting to me is the question of what kind of build each of the PMAS characters would actually
make, if we were going to run a game together.
Sabrina would... probably be the DM, actually, and use grief control to make a sweet map and minis for everyone. If she did play, she'd make a ludicrously OP cleric build, but then just sit back and support everyone else and not use the build to the full potential, so everyone can have fun.
Mami would probably make a burly orc barbarian and have fun charging at people and rolling huge piles of d12s.
Sayaka would, indeed, play a righteous and valiant paladin and protector.
Homura... would have to be kajoled into joining, and would only play if Madoka was there. She'd get Sabrina to build her character for her, and Sabrina would meddle as usual and give Homura a character with good synergies with Madoka.
Madoka would go way overboard on her characters backstory, then try to peacefully resolve the conflict between the two warring kingdoms - and actually succeed because of her crazy diplomancer bard build.
