The overwhelming obstacle here is electrical consumption. I imagine that bitcoin mining on a mobile device will be incredibly taxing on the battery of said device, rendering such an activity very hard to perform when the mobile is not docked to a charging station. If this is the case, then the mobile will be at best a USB miner that you can plug in for 8 hours at night when you sleep.

Edit: Also, it would not be unfair to assume such laborous usage of your mobile's battery will lower its life expectancy in the long run