They probably wanted to show that he wasn't exactly comfortable with the times, no? The real Graysmith did also comment about the awful clothes her wore in this period. They didn't look that bad, but perhaps they were so different from the trends in the 70s...

So I'm thinking that RG
did wear some awful 70's clothes, but for whatever reasons, they didn't have Jake wear them. The clothes he wears in the movie are just plain, sensible clothes, not the trendy 70's stuff. If RG said the clothes he wore were awful, I don't think he would say that about the clothes Jake wears in the movie.
Sorry everyone, being OT for this thread.
kathy