Its not very efficient nor is it rewarding unless your luck factor is insanely good.
The number of computations your mobile phone can do efficiently is certainly a lot less than a dedicated mining rig.
To conclude, it is possible but just not worth it since your odds of successfully mining a block are very tiny however you can use your phone as a master to a number of slave mining rigs somewhere else to make it more efficient.
How about your luck unfavor you and your phone exploded because of the heat that your phone emits? And how can it be efficient when your phone can't do what the GPUs can and your phone will probably lag. I mean our phones isn't design to mine and just forget it or you'll just keep on buying a new phone, then what's more profitable or at least a wise move?