It's the other way round from a cause & effect perspective: the worse the user interface, the more off-putting it is to the casual users that create and promote low-quality content.
The old reddit user interface was/is remarkably good. Threaded, collapsible discussions with markdown for comments in a non-distracting (i.e. sparse) layout.
Once you found the comments it was but the comment link was/is marginalised in favour of the link itself, making it easy to miss that they're 'the point'. Even before new.reddit it was a common complaint that it's confusing to new users.
Obviously this is relative, it's a huge site! And there may even be value to gatekeeping users who can't figure it out.