Season 1 and 2: dull and too serious?
Pretty much when Homer had his Walter Matthau voice feels like when they were finding their feet. Don't get me wrong I love that "Do the Bartman" era too, but Season 1 and to a lesser extent 2 had a few gems and decent episodes mixed in with some that just didn't seem that funny or entertaining compared to after that.
They also kinda appeared to be attempting to be more of a serious family sitcom too. A few eps were just flat out depressing and sad (like One Two Blowfish Blue Fish where Homer thinks he's dying, or Bart Gets Hit by a Car).
Seems like the legit Simpsons really began in Season 3, and Homer got more goofy. I'm not sure if Groening just let it loose realizing they could get away with more by then (and knowing it was a hit), or if they just perfected the writing and the characters. MANY shows seem that way - primitive in Season 1, almost but not quite there by Season 2, and 3 is where they hit their pinnacle.