When did it start to suck?

In this case it was because the original creator Genndy Tartakovsky had left to work on Samurai Jack, apparently Chris Savino had decided to change the character designs and backgrounds drastically because there were comics that had that look, shortly into this run Christine Cavanaugh the voice of Dexter decided to leave show business and was replaced by Candi Milo who wasn't nearly as good, and they had new writers.

Seasons 1 & 2 were the good ones only. This happens a lot with cartoons that are ended and then revived a few years later. Examples: Powerpuff Girls were good for seasons 1-3. Season 4 wasn't bad. 5-6 I don't even consider. SpongeBob SquarePants seasons 1-3 were great. A few hit or miss or episodes in the subsequent season and since then it's been a *beep* fest like every other modern Nick show. Rugrats was good 1-3 and had many good follow-up specials that then became a part of season 4. The show drastically changed but still remained good here and there until Dil's arrival though it wasn't until Kimi showed up that the show became absolute *beep* Courage the Cowardly Dog was great 1-2, 3 was great as well but featured a few episodes that showed declining quality. Season 4 had its fair share of great episodes but was considerably different and not in a good way overall. Almost every incarnation of Scooby-Doo post 'Where Are You' and 'The Scooby-Doo Show' was hit or miss in its episodes. Some were great and some awful. 'What's New' is when they really made drastic changes following the live action film and while it had a few decent episodes it was mostly awful and every subsequent series has been even more awful. But this happens with sitcoms too. The Simpsons have declined in quality since the early 00's, Supernatural should've ended with season 5, and the X-Files should've wrapped up with 7 following Mulder's departure. Sometimes people get money hungry and we see beloved series' become something different altogether. Our only choice is to pretend they don't exist.

Pretty much the moment that hideous animation hit the tv screens. It really rubbed me the wrong way as a kid.