It can target everything that C89 can target, but it may require some additional steps like writing the Nim interface code for target specific system libraries etc. There are also tools to help with this, but I prefer to write the Nim interface to existing C code myself.
Also have a look at the following list:
https://forum.nim-lang.org/t/2670
(nimx, a cross-platforn GUI library, seems to support Android/IOS)
The following shows how to interface with ObjectiveC code / libraries:
https://github.com/jangko/objc
(Of course, if they are not already available, you will have to create the Nim interface to the IOS libraries yourself)