There are a few reasons why I believe this won't work.

1. The heat produced from it. You could not hold it in your hand because you would actually be burned, so the smartphone is rendered useless as you could not hold it. Plus the noise it would make.

2. You would get an insignificant amount of Bitcoin. Your phone would mine so little that it wouldn't be close to worth it. I have a few miners hooked up for fun just to get a few thousand satoshis worth of crypto every now and then. But with a phone I estimate it wouldn't mine any where near that amount.

Cool idea, maybe portable miners would be cool. About the size of a raspberry pi that you could take over and show to your friend run off of a battery that charges like a phone. But having a phone be a miner, not yet.