By installing xCon on your iPhone, the Miitomo app will function properly. However, this requires you to download version 40 or 41 specifically. Earlier versions will not run the app and will result in the same issue that iPhone users are already experiencing. xCon can be downloaded by following these directions on your jailbroken phone.
Some users are also reporting that Tsprotector is also acceptable as a means of getting Miitomo to run. However, Rii__ has stated that they have not confirmed this as true, so xCon seems like the way to go.
Another issue that is overlooked is users that have an iPhone that was previously jailbroken but is no longer. Since there may be jailbreak files left over, and the anti-jailbreak apps are picking them up, you must complete a Default Firmware Update (DFU) restore in order to run the app. The instructions on completing a DFU restore can be found here, but remember to backup your files or else they will be deleted.
If you are still having issues, you can join the ever-expanding thread on Reddit and ask any questions you may have about your jailbroken device.
