The appeal lies in the contrast between the characters. Cala Maria is this powerful, otherworldly being, while Mugman is a simple - looking but endearing cup. It's like mixing the extraordinary with the ordinary, which can create a lot of interesting scenarios. Their interactions can be full of surprises.
One big part of the appeal is the potential for character growth. Mugman could grow more confident in his relationship with Cala Maria. He might learn to stand up for himself more. Cala Maria could also become more empathetic towards Mugman. In fanfics, we can see how they influence each other in ways that the original game might not have explored.