Thanks Hannah. It's true that a joke stops being funny if you have to explain it. As such, are memes impossible to alt text without killing the joke? I had a similar debate over on LinkedIn.
At a fundamental level, I think it's sad that visually impaired people may be excluded from enjoying memes. But as Jodi points out, they're often 'inside jokes' anyway, which exclude some people by design. You either get them, or you don't, often based on your familiarity with the pop culture they reference.
But I do think it would be entirely possible for screen readers to pick up a '[sarc]' tag in the future, and adjust the tone of its delivery accordingly. Just as WellSaid offers narration, promo and conversational tones.
So who knows? I hope memes can become more inclusive in future.