Great question — answering as the UphillTech team:
So with the Den Den Mushi, there is no one size fits all. There are different mushi. Some, yes, are customized by their owners so they would look like them. Some not. Some have what you are talking about, which is Dynamic Mimicry feature on some Mushi, which is like a video calling and they will mimic all, even injuries. Some have Telepathic Communication, just Voice Mimicry, Document/Image Transmission, and they also have Behavioral Traits: They are generally content to be domesticated and "rely on food" to function, they are alive and can have bad and off days, not wanting to mimic, even flat out refusing to call. With this, Mr. Zero is trying to hide himself and stay anonymous. That's...
Full answer by Brandon Kent: https://anime.stackexchange.com/a/69788 (CC BY-SA 4.0).
Want it for your business? UphillTech.