The minimum level of pollution where Mars looks good on that basis is:

1. killed literally, not metaphorically, everything

2. and literally boiled off all the oceans, rivers, and lakes, top the bottom

3. and strip mined literally, not metaphorically, everywhere to a depth of 3,000 km

4. and removed almost everything from the atmosphere except the current level of CO2/m^3

Even a "polluted beyond help" Earth would be easier to live on than Mars. We have liquid water, a nice thick atmosphere that isn't 95% carbon dioxide, a better ionosphere, easy access to nitrogen as a synthetic fertilizer feedstock, and various other advantages. Musk knows this; he's not an idiot, nor is he a cartoon villain motivated by a desire to be Evil.

It bothers me when people just make up outlandish accusations.

Even assuming an absolute worst-case scenario climate-wise (no mitigations whatsoever, massive sea-level rise, etc.), Earth will still be dramatically more hospitable to human life than Mars for the entirety of Musk's lifetime, and very likely his children's and grandchildren's lifetimes as well, barring some mass-extinction event like a nuclear war. Even if a colonization effort begins soon, living on Mars is likely to be extremely difficult for centuries -- we can't breathe the air or grow anything in the soil, our biology is likely ill-adapted to the gravity, there is no liquid surface water, etc. -- and the idea that there will come a point in the foreseeable future where it makes sense to escape there because it's easier than being here (because of pollution or whatever other reason) is difficult to fathom.

I guess it's always worth having a Plan B, but Nate Hagens and Kim Stanley Robinson (sci-fi writer of the Mars Trilogy) discuss this and don't necessarily agree, obviously depending on what definition of "polluted beyond help" you're using etc. [1]

Nate Hagens (00:43:15):

"And to me it's very smart and famous people, Stephen Hawking, Elon Musk among them say, "We're screwing up Earth. Climate is going to be bad. We need to colonize outer space and head to Mars." And that makes just zero sense to me, because on the worst thermonuclear runaway-climate scenario, Earth is still going to be a paradise next to Mars. I mean-"

Kim Stanley Robinson (00:43:43):

"I agree with you completely. That is a case that has to be made. And the thing about people like Hawkings, the thing about people like Elon Musk is they're smart, they aren't that smart. They aren't wise, they aren't philosophers, and they're not ecologists. Very often physicists will assume that ecology is just physics in action, and that they've got it all sussed out because they know the rules of physics and they can do rocketry. But you can't do plant biology just knowing rocketry. It's more complex, it's more interconnected, it's a more-complex science. And many physicists will arrogantly say that, "Because I know the laws of physics, I know everything." But they don't know how a marsh works. They haven't studied biology enough to understand their own ignorance. So how smart are you if you don't know how ignorant you are?"

[1] - https://www.thegreatsimplification.com/episode/66-kim-stanle...

We're one killer global war / pandemic / etc away from extinction

Being on multiple planets (and eventually multiple solar systems) prevents a species from going extinct if something catastrophic happens to their home world