Basically. SA2 was the turning point for the series in a lot of ways.
Anyone remember sonic-anime.net? That was the main forum I went to before Shadow came out and I also visited a lot of Sonic fansites during that time. They were mostly civil but there was some underlying pessimism because things like the 3D games, Sonic X, the jump to multiplatform, and Shadow himself created some division. It wasn't that bad but it was a pretty standard mid-2000s internet community, relatively.
But really though, things really started to go to hell once Shadow and '06 came out and people either stuck around or jumped ship.
SA2 and the anime created the new generation of a weird fanbase.
I joined the online Sonic fandom back when Sonic Adventure was released, going to The Sonic Foundation and Sonic HQ in particular, and I remember the environment being pretty good. There was some creepiness, I suppose, but it was mainly restricted to the SatAM cartoon and comic fans, as I recall, and I stayed away from the parts of boards dedicated to them. When Sonic Adventure 2 came out, I definitely recall a lot of people creeping on Rouge and such, but my favorite memories of forum RPs and group fics come from after its release.
I was going to say that I left the fandom shortly after Sonic Heroes came out, but that second post reminded me that I actually did continue posting on those forums all the way through the first 52 episodes of Sonic X (subbed). Oddly, not many people on those forums actually talked about it much.
I know for a fact that Shadow the Hedgehog came out after I broke my ties with the Sonic fandom, though that was more because activity at The Sonic Foundation died down so much. This probably does relate to the pessimism felt by people about the direction of the franchise and especially Sega going 3rd party, looking back. Even when Shadow came out, I thought it looked ridiculous and didn't even consider buying it. When Sonic 06 came out, I was salty that it didn't come out on the Gamecube (which I basically bought for Sonic Heroes) and thus was glad when it turned out to be shit.
It is amazing how much Sonic Adventure 2 changed things. At the time, it felt like Sega only had to make a few adjustments before they reached the perfect formula for Sonic games. Everyone was really interested in where they were going with the entire Ultimate Lifeform thing and the backstory of the Robotnik family, and I would often write down my ideas of what Sonic Adventure 3 would be like during class (instead of taking notes). Sega going third party soon afterward sent everything going down the drain for the fandom.
EDIT: Made myself sad looking for The Sonic Foundation, seeing it's no longer up, and then scouring the Internet Archive for old forum posts only to find that it doesn't have access to the forums after 2001.