I feel like it could work. If you think about it, you need the cost to the client to be greater than the cost to the server. As long as that is true the server shouldn't mind about increased traffic because it's making a profit!

Very crudely if you think that a request costs the server ~10ms of compute time and a phone is 30x slower then you'd need 300ms of client compute time to equal it which seems very reasonable.

The only problem is you would need a cryptocurrency that a) lets you verify tiny chunks of work, and b) can't be done faster than you can do it on a phone using other hardware, and c) lets a client mine money without being to actually spend it ("homomorphic mining"?).

I don't know if anything like that exists but it would be an interesting problem to solve.