Yeah, Arthur was a big part of my childhood. I would say the first 9 seasons hold up very well. Seasons 10-15 is where you start to see signs of decline and than Season 16 is when it completely went off the rails. I'm still interested enough to check out the new season, and it's just 3 episodes long anyways.
Arthur had its peak in the late '90s and early '00s, then the show itself just became a product of the time it was made (mainly for its Doug-like influence), and was largely forgotten by many... until the memes happened.
Seasons 1-4: Gold standard, definitely when the show was in its prime.
Season 5: Despite some of the original key creative people still involved, Arthur's voice drop was disorienting.
Season 6: My PBS skipped this season altogether for the longest time, but once I did finally see it (via On Demand in 2004), it was incredibly weak.
Seasons 7-8: Definitely an improvement over the previous couple of seasons with a number of memorable episodes.
Seasons 9-15: Weak, unbelievable stories and miscast voice actors, never bothered.
Seasons 16- 18 1/2: I mean yeah, the Flash animation was definitely off-putting at first, and the Compsons were very polarizing, but I wouldn't say these were either bad or good . . . plus, the return of one of the original writers from Seasons 1-4 definitely brought back a sense of that classic ARTHUR feel.
Seasons 18 1/2 - Present: Just grew weaker and weaker, and like Any said, don't care anymore.
Funny thing is, the first three seasons are the only "long" seasons the show had . . . I know Season 1 had 30 episodes, I think both Seasons 2 and 3 had 15, then from Season 4 onward they were 10 for the longest time.