>>95806142Season 5 is when Kevin Sullivan started writing episodes, no surprise a lot of people point to that as when the slump really began.
The thing is different characters started becoming shitty at different points in the series.
>Cosmo and WandaCosmo became a complete retard and Wanda a nag by season 4.
>TimmyWas always something of a selfish kid but it really got out of hand in season 5, to the point where they had to address it in Fairy Idol
>VickyBecame an over the top, world conquering villain whose own parents feared and hated her around season 4, specifically during and after Channel Chasers.
>JorgenWent from an intimidating hander out of punishment to a completely retarded joke who lets rules get broken in his face around season 7.
>CrockerBecame an over the top villain after his origin movie and Abra Catastrophe, took over the show entirely after season 6.
>TrixieBecame a slut and a bitch after season 2.
>FrancisLost relevance and disappeared around season 5.
>TootieBecame uncharacteristically violent in season 7 then disappeared completely afterward.
>Chester and AJBecame total assholes to Timmy around season 4.
>Timmy's parentsWere never meant to be likable characters to begin with, but Timmy's dad started getting shit tons of episodes focused mainly on him in season 7. Timmy's mom had a few but mostly disappeared from the show or was regulated to the background.
>Crimson ChinStopped being a thing after season 6 Goldenlocks episode, sparing mentions and cameos aside.
>CatmanVoice actor changed from Adam West in season 9 and completely fucked up the point of the character.
There are tons of other characters I'm probably missing or are irrelevant (Sanjay and boil kid), but the point is that in a heavily character driven show like this, having so many of characters you like disappear or become flanderized ruins the joy of watching the show. So its hard to pin it on any one season, but 4-5 are when the most characters went to shit.