Since the waste product is heat, and if you live in a place where you need heating most of the year it's simply shifting heat generation from your heater to your phone.
In some places where heating is still done with natural gas you might even be more eco-friendly by having an inefficient charger.
Sure, there might be some niche corner case where it actually pencils out better. But that's not really helpful when assessing the impact of this on a global scale. There's some other scenario that's the opposite that cancels the benefit out.
And as far as heating is concerned, heat pumps are more efficient, and most electricity—when turned into resistive heat—is less efficient than burning the fuel directly.
So the only case where it would be a benefit is if your heat is fuel-based, but your power is renewable, and this condition lasts most of the year.
In some places where heating is still done with natural gas you might even be more eco-friendly by having an inefficient charger.