I don't browse Reddit unless I was linked there, so I might not have seen all it has to offer, but my impression is that it's far more "fluffy" than HN.
Reddit's default theme is more or less as spartan as HN, it just uses more JS for interactivity. However, each sub can configure their own theme, which tends to lead to geocities syndrome.
(If you're registered, you can disable themes, presumably for that reason.)
If brutalism is favouring functionality over "design", then they are some of the most popular examples I know of.