The theoritical megalomaniac ability of terraforming Mars, to escape from our theoretical future inhabitable Earth, is a contradiction in terms.
If we would be that much powerful to terraform Mars, we would not need to do it. Because we would be powerful enough to save our own planet, or even change the Earth's orbit in case of a potential collision with another heavenly body.
If we don't learn to live here, we don't learn it anywhere else. Also, my personal conclusion is, that anyway terraforming Mars is not possible by no means. It's conditions do not even equal the conditions on top of Mount Everest, not even equal our Antarctica. Temperatures on Mars partly change by about 100°C within just one day.
We will die anyway. Also on Mars. Nothing lasts forever, less than ever we insignificant creatures within this solar system. Let's stick with our blue pretty planet. We only have this one, which by the way did not get ruined, nor is it going to be overpopulated. The population increases less than expected earlier, while this is going to turn into a decrease past 2050, in case the birthrate per women should be around 1,85 as predicted by the UNO. Also, overpopulation is not a global problem. It takes place in developing countries basically.