I already saw some devs doing for iphone, so it can be done...
But honestly i don't know how to
Andiod .apks anyone can make and install on (pretty much) any Andriod device.
Iphone .ipas anyone can make BUT cannot be installed by everyone.
ONLY Jailbroken or dev enabled iphones can side-load user created apps.
(if you're jailbroken but don't know how to side-load, you got bigger problems)
Looks like renpy has documentation on the IOS process:
You must be registered to see the links
IMO I think it would be SMARTER for everyone if someone was able to create a Renpy application that can be used to play renpy games, rather than create 1 game per app. But still, it would have to be approved by Renpy and then Apple so that jailbreaking isn't required.
BUT it would require A LOT more work than just porting the game to ios, since the whole renpy library would need to be nested within xcode and then have some type of translation layer for it to work correctly. AFAIK xocde doesn't play nicely with other languages.