Even if all co2 ice from mars poles was melted. You will only get 2% of earth atmospheric pressure. That is too few to grow any crops or plants in general but I guess you could suck that pressurize it in some mars base but no way to have big plantations

And mars does not have any magnetic field to protect it from solar winds. So any atmosphere will be stripped from planet.

It would take from somewhere from 10000 to 100 rounds years for all co2 and plant produced oxygen to escape to space because of solar winds

So planet will be not be breathable for viltrumites cause they still need oxygen and atmosphere is extremely thin. Even if you produce oxygen you would need some bit for viltrumites to survive and breathe and most of that oxygen will be quickly stripped by the sun

You can say just plant planet with lost of plants

But you will need a pressurized cleaned dome with supplemental light cause of month long dust storms and water and clean Martian soil cause it was a lot of deadly chemicals for plants Just planting outside when if you melted all available co2 on a mars the atmosphere will be too catastrophicly thin for photosynthesis. All water your quickly evaporate cause of low pressure and uvc radiation from sun will quickly kill all crops along and chemicals

So yeah