Create and organize your campaigns in the way that will benefit you. If there's a good reason to separate campaigns based on mobile and desktop devices, do it. For instance, if you were a taxi service, your customers may be different if they search on their mobile than if they search from a desktop. In this case, you may want or need to target these segments differently with different ads, maybe even different keywords.
There used to be a way to simply say a campaign was either for desktop, mobiles or both. Adwords evolves but sometimes it seems not always for the better. The workaround is to place a lower bid and increase the mobile bid by a high percentage. The maximum you can enter is 300%.
So if the first place bid is $1, decrease it to around $0.30 and increase the mobile bid to 300%. This won't prevent every impression on desktops but it should decrease it quite a large amount, effectively making that campaign mobile-only.
Like I said, you would do this only if a good reason to do so. I've rarely found that to be the case and typically, my campaigns turn mobiles off because of the poorer results. But each case is different. And if you do, don't just copy everything over. If you use the same ads, what's the point?
