The problem with that argument lies completely in the term "Can't"

We literally "CAN" take care of this planet. If we were willing to free ourselves of convenience, financial gain and change our lifestyles. We have the ability to ban, outlaw, prohibit and reduce use most of the things that we do today that are aiding in destroying the planet.

Corporate profits are essentially the number driver behind non-compliance or non-decisiveness to care for the planet.

The reason I bring that up is that it is very important in noting that "If we actually CAN take care of this planet (Just choose not to) then we actually CAN help terraform Mars."

Given what I have observed in my time on this planet, we have technology, and have the ability to develop technology that can do a great many things... I would not put terraforming a planet out of the realm of being achievable. In a dedicate world, with a unanimous mindset on a common goal.