Who cares if it's not economical on commodity hardware, be it a smartphone or not. If I have a website that is visited by enough people to mine me a (part of a) bitcoin, while at the same time the cost of hosting the site is less than what is earned, what does it matter if the mining power is not efficient? If I care more about money than about environment, it's a good business model. But you probably need a lot of visitors for that, and I have no idea if it works out.