Well, I think the immediate answer to your question is that mice are adorable. They’re really cute little guys, right up to the point where they move into your house and start nibbling on everything and pissing and pooping everywhere. Also, mice are ubiquitous – you’re going to find them in just about every household.
What’s the alternative for happy little Cinderella helpers? Cockroaches? (In Rumplestiltskin, as I recall, it was the ants that came to the rescue.)
But really, you * don't * want to start thinking too deeply about the implications of a Disney universe where all the critters are intelligent, cute, and verbal. The Little Mermaid's dinner scene lampshaded the whole disturbing "Hey, these are intelligent creatures and we're eating them" issue.