Well, that's just it. She was created and crafter after Dante's mother. That's the face she was born with, why would she want to look like anyone but herself. That and I'm pretty sure (not 100%) that Gloria is just an outfit a wig and a tan job, her face didn't actually morph.DMC4 retconned Trish into a shapeshifter. She only looks like Dante's mother because she feels like it; she could look like anybody, and there's no telling what she looked like when Mundus first created her.