Oh he was fun enough, with some very funny lines and scenes and a fantastic death scene, and the design is very meme-y and iconic. I probably wouldn't feel let down at all if I'd just binged watched the show after it aired.
The thing is just that he was at the center of a lot of the codes and hints that went along with the show in its meta-game. Plus something like the reddit AMA, which was funnier and weirder than the show because there were no Disney censors to stop it from going to weird shit places.
It's just that in the end, Bill wasn't a particularly clever or competent bad guy. It turns out that he really did lose twice to a few kids instead of having any larger plan, so he was really just a failure from the beginning. It makes sense that he's overconfident and lazy with nothing to back up his ego, it just made the finale less tense because it seemed like he didn't know what the fuck he was doing. He was a frat boy fucking around and partying which is a funny idea, but not a very tense one.
I think Bill's best scene was probably when he tricked Mabel, and shit seemed to be getting real. But that gets scaled back immediately. It's not the End of the World but just an annoying local thing that happens for a few days. Mabel doesn't suffer any fallout for that emotional moment at all. I'm not saying she needs to suffer, just that the stakes were low. So the hype from the end of DaMvtF kind of petered out.
He was still a funny bad guy for a comedy show and even in the finale had a lot of great scenes. But in terms of being scary, there were lots of other Gravity Falls monsters who seemed like bigger, more competent threats. And the early idea of a character making fun of conspiracy theorists would probably be funnier on a show for adults that allows not so G rated humor.