Even if the electricity used to power the Tesla is 100% powered by coal, it is still more environmentally friendly. Solar is dropping year by year and will be cheaper than fossil fuels. So, in 10 to 20 years, the plurality of power will solar generated.
I will say that base load plants are running at lower utilization at night and could be enhanced with more kWh draw of EVs at night during the lull in demand. If demand for refined fuels drops, then what you may want to shoot for is a lower number of refineries. The amount of pollution in Louisiana is high surrounding refineries. The coal plants out there could run more efficiently at night if they didn't need to increase the grid voltage and frequency due to lower demand. A few million EVs charging at night is practically a good thing even with coal plants running as much of the base load. In texas a lot of the windpower generation is at night.