Dull is an adjective that has many different acceptions.
In my context, I deduce it must mean ‘blunt, not sharp’ which talking about hair it’s somewhat odd, even if it’s talking about little buns of hair I presume, but it’s the only sense I found that makes a little sense. It’s not talking about the colour, as it says it’s golden and shiny, so that cannot be.
Something that can be sharp, like a blade, makes sense to be dull, but I can´t think how any hairstyle could ever be sharp…
“her hair, no longer white, but of a rich golden colour, streamed like a cataract, here falling in dull gathered heaps, there rushing away in smooth shining falls.”