I'm with Amberlee, I don't think it's Ritzy at all. I think the mice are pulling one over on you. They are really real mice that are feasting on poor Ritzy's food and drink. However, they don't want to be exterminated by you, so when they see you coming around they all turn themselves into fake looking mice (the aliens taught them how to do that) and laugh behind your back as you go looking for Ritzy and some sort of explanation.

Maybe she likes company while she's eating - similar to the hostess with the mostest theory.

That Ritzy is such a character.