After thinking more about this, I don't think smartphones would even be good sources of ambient data that could improve forecasts.
Smartphones are personal computers. They spend most of their time in pockets and controlled indoor environments. This ambient data is of no use to anyone, which is why there's still a market for home weather stations, whose sensors are typically placed outside.
The barometer data is for sure noisy, and must be cleaned and quality controlled. But that is possible to do, has been for 10 years now (there are published papers and demo apps that can do it). For one, rate of change of atmospheric pressure is pretty much the same inside as out, your main challenge for the raw value to be correct is user elevation. That can be corrected in quality control as well.
Plenty of work has been done on this front, and it can be demonstrated that you can assimilate the smartphone pressures into weather models and get some good results. It is hard, of course, and I’m not sure personally how much better the forecasts get. But it’s absolutely possible.