Not a bad idea. I found the Unicode glyphs for "Squared Plus" and "Squared Minus" and used them instead. They needed to be scaled up a bit but after that it was fine.
This latest version of Hacker News Comment Hiding is available at the same URL as before, https://userscripts.org/scripts/show/130027 .
One comment: the triangle gets a bit confused with the upvote/downvote buttons. Maybe it'd be clearer if it was something like [-] / [+]... it would also enlarge the clickable area a bit, probably.