Copy on write is an interesting idea used on server systems. Does it make any sense on a smartphone?
Yes, copy-on-write makes sense on smartphones, too.  Smartphones are just computers, packaged to be small.  Smartphones run conventional operating systems, with multiple processes.  For instance, each app typically runs in its own process.  And yes, these processes do use shared memory, shared libraries, fork(), etc., so copy-on-write is useful (and is supported on typical smartphone operating systems).