I think that Darcs did help draw attention to Haskell, just that it's still not especially popular. The fact that we're talking about Haskell and not e.g. Joy says something about it at least crossing a low threshold of popularity, though.

(XMonad is a much less significant example for Haskell's popularity, IMHO.)

It seems highly likely to me that Haskell's type system made inventing Darcs's theory of patches significantly easier. While a Darcs-like system could be written in C, designing the system as a whole was almost certainly aided greatly by Haskell's type checking and lightening of certain conceptual burdens through laziness.