Tankies don’t usually believe that Stalin or Mao “did nothing wrong,” although many do use that phrase for effect (this is the internet, remember). We believe that Stalin and Mao were committed socialists who, despite their mistakes, did much more for humanity than most of the…
I agree that memes like "Stalin did nothing wrong" are unlikely to convince anyone, but they're not really designed to. It's just a way to state you're not going to be drawn into an argument that you believe is in bad faith, and maybe give other online communists a chuckle. Although I have known a couple of people who saw that kind of statement and found it such a shock to the usual discourse they actually went and did some reading on Stalin, something they'd never done before.
It's up to each individual comrade when they see the need, benefit, or can be bothered to engage and educate. I also don't think there's that much downside to comrades occasionally just giving that kind of glib response. Amongst the enormous tide of an anti-communist western hegemony and incredibly extensive state and capital funded disinformation networks, it's a homeopathic level drop in the ocean of what might turn people off.