@hedge, you're asking if we should terraform Mars if we haven't already cleaned up this this planet. It's a good question but I don't see a problem here.

Let me borrow a quote from Isaac Arthur, youtuber and president of the National Space Society(in USA), and I'm paraphrasing him: If we have the technology to truly terraform Mars, then lot of that technology will already have been used to stabilize the climate on earth. It's by orders of magnitude easier to "fix" Earth, than make Mars habitable to humans without the need for Domes, or spacesuits to breathe outside.

So to continue the "cleanup" analogy, it's like cleaning up the worst nuclear disaster (Chernobyl ) vs cleaning a few drops of water off your kitchen floor.