That's an incredible waste of a lot of computing power.
The average program, even on a PC, barely comes close to utilizing the full computing power of a CPU core. More often than not, a CPU core is doing nothing, idling, etc.
Take my desktop PC for instance, an Intel Core i5 CPU with 16 GBs of RAM. 99% of the time each core's utilization percentages sit below 10 to 15%. And I've got a lot of programs running but still CPU utilization is very very low.
Now, take your typical smart phone app. Those apps pale in comparison to the average desktop app in terms of complexity and capabilities. So yes, having a CPU core dedicated to an app would be incredibly wasteful.