[schwarrrtz]

Carbon neutral assuming you are creating the CH4 using atmospheric CO2. On Earth I suspect that most CH4 comes from natural gas. On Mars it would be carbon neutral - but that's actually a bad thing, assuming that your goal is to increase the atmospheric density on Mars.

[perilunar]

Yes, that was the assumption of the parent question. I expect that initially all the fuel for Earth launches will be sourced from natural gas, but if they start doing regular sub-orbital passenger flights they'd have to convert to renewable energy + atmospheric CO2, otherwise they'd just (partially) undo the work Tesla is doing.