The 21st century mr. Potato head! - ?
I like the overall design aesthetic, it's a friendly mix of organic and mechanical elements. Maybe the attachments could be more convincing and fun. Most of all I think the app as described is a bit limiting. This will probably make or break the entire concept. You should probably develop some concepts for what else the app could do with some more intelligence behind it. Recognize your face? Recognize emotions and reflect them? Interact with other creatures when you put them together? etc.
I would consider buying something like that if it were also useful for something for me, because I like the fun aspect and the design. But as a phone case it's not right for me.