Well, a smartphone is fairly bad for its small size, but we're talking less than 100kg of CO2[1][2] to make and operate for its lifetime. But driving a car for a year in the US produces 4600kg[3]...
My understanding is that CO2 output is the least worrying part of smartphone usage. It's all the other toxic waste released during mining. Also, if cars are more polluting, that just means cars are bad too, not that smartphones are magically ok.
https://phys.org/news/2018-08-ways-smartphone-environment.ht...