The effort is minimal, and I don't think most people choose whether or not to join a site based on how pretty it is. Just look at Craigslist for instance.
>but it certainly drives away many of the people who would otherwise make low-effort submissions.
I don't think there are actually a lot of people being turned away from Hacker News because of the layout who otherwise might join if it had more features. Remember that the entire pool of potential users for Hacker News includes anyone who even knows that it exists -- this is not a mainstream site and it doesn't have a lot of exposure outside of technical circles. No one browsing the main subreddits or Twitter or Facebook is likely to ever see it even mentioned.
That an "ugly" layout keeps low-effort users away seems to be a common belief and I think it's one espoused by pg, but the claim seems a bit like that made of a tiger repelling rock ... the only "evidence" that it works being the absence of any tigers.
Although... Hacker News has plenty of low-effort submissions, and people who complain about it "turning into Reddit," which was once cited as a "common semi-noob delusion" in the site guidelines, it's such a common occurrence.
IMHO HN looks and feels a way better than Reddit and Facebook is just complete crap.
The same thing goes for HN. Why would you go through the effort to make an account for an ugly site when there are better looking options available (Reddit, Facebook, etc)?
Yes, it doesn't have much to do with intelligence, but it certainly drives away many of the people who would otherwise make low-effort submissions. In fact, I wish it were even _more_ barebones (e.g. remove the user points in the corner).
I used to think HN should have more features, but then I used Reddit and came back wanting fewer features. I like how it's simultaneously inclusive (anyone can create an account) and exclusive (somewhat ugly, no memes).