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.

A thread about the added Android support: https://forum.nim-lang.org/t/3098

An old article about targeting IOS: http://www.thomasdenney.co.uk/blog/2015/1/27/nim-on-ios/

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)

One of the tools that can be used to create Nim interface code from existing C header files: https://github.com/nim-lang/c2nim