I found a couple of articles, a 1970 Time magazine article and another 1970 article that appeared in a short-lived street fashion magazine called Rags, that referenced musicians other than the Grateful Dead, and used that as justification for a change in the Wikipedia article.
The Grateful Dead, or more specifically their followers, were huge in the popularity of tie-dye in later years, but these contemporary sources don't even mention them. I think that the Grateful Dead were not necessaily among the
first popularizers of tie-dye in the 60 and 70s. There's even a
1994 quote in the New York Times from Bob Weir saying that he had always hated tie-dye.
(I've posted excerpts from these articles—including the whole Time article—in a long-running section of the Dye Forum on my site that is titled
Tie-dye history in the West - 1960s influences?.)
-Paula