Honest question, did anyone unironically think that happy tree friends was good? To me it was just one concept (horrific gore portrayed in a Nick jr styled aesthetic) stretched out for too long.

They were SWF animations – a decent compromise of quality and filesize. Those were popular at the time, beats streaming 144p Youtube.
As for the theme, well, it must have been a novelty at some point. Being edgy was the shit back then wasn't it. I know I was into it as a teen.
With mute, basic characters it couldn't have been much more than a one-trick pony. Though, I bet it inspired some of today's content providers, who are just better at using that old concept. You know, it's like this "Seinfeld" Is Unfunny thing.