Yes, a vegan diet reduces harm. So does biking instead of driving. So does making your own clothes instead of buying sweatshop clothes. There are LOTS of ways to reduce the amount of suffering your life causes. I wouldn't exclude anyone from being a leftist because they don't do all of those things.

If you're not doing absolutely everything you can to reduce your impact on the planet to the bare minimum, regardless of your happiness, then you can stop shitting on the rest of us for not adopting your particular form of harm reduction. And to be clear, I'm using "the royal you" here - I don't know how you, Lemmee, treat the "carnists" in your life. But I certainly can guess how the person who made this meme does - and that person is a hypocrite.