First, a consideration from the early 1990s:

A variant of meh, namely mnyeh, famously appears in the works of the late academic and humorist Leo Rosten, with extended treatment in his Hooray for Yiddish. But Rosten’s work is often disparaged by experts in Yiddishology, and, in any case, there’s no evidence that mnyeh is even a Yiddish word.

The British poet W.H. Auden didn’t think much of the first lunar landing, and he wrote a poem about it (but used “Mneh”):

Worth going to see? I can well believe it.
Worth seeing? Mneh! I once rode through a desert
     and was not charmed: give me a watered
     lively garden, remote from blatherers

Finally, the tracing of “meh” to The Simpsons:

Though Swartzwelder is the most prolific of all Simpsons writers, he has led a reclusive lifewriting novels since leaving the show in 2004. After finding a way to send him a message, I was amazed to actually get a response from the man called “the J. D. Salinger of comedy writing.” He didn’t recall putting meh on the show before his “Hungry Hungry Homer” episode, which aired in 2001, some seven seasons after meh began to be used by Simpsons characters. Oakley, though, suspects meh might have appeared in an early Swartzwelder-penned scene that never made it to air. And Swartzwelder did have a memory of where he first came acrossmeh, though it wasn’t in Mad. “I had originally heard the word from an advertising writer named Howie Krakow back in 1970 or 1971 who insisted it was the funniest word in the world,” he told me. So let’s thank Mr. Krakow for his unwitting role in the spread of the meh meme.

Wikipedia has this to add about “Meh“: the word’s first mainstream print usage occurred in Canadian newspaper the Edmonton Sun in 2003: “Ryan Opray got voted off Survivor. Meh.”