As New Scooby-Doo movies go, this one is decent. The gang are all great and there's a very funny setup of Winters, Shaggy, and Scooby quietly wrestling for the position of most cowardly. I also laughed at the extended water wheel scene - an underrated slapstick set piece!
Still, a real risk of these kinds of guest star focused episodes is one of timeliness: 40 years on will we remember who the guest stars were and why the jokes they told were funny? The answer for this episode is... kind of a resounding "no". I had no idea who Jonathan Winters was, and don't think I've ever heard any of his old routines.
From context clues I assume that Ma Frickert was a recurring character that Winters would do in his comedy, and I'm sure there's a long catalogue of hilarious Frickert's Farm jokes that presumably viewers in 1972 would have been familiar with, but watching in 2020 it's all sort of baffling. His stand up style of doing voices is also fairly dated, but that scene at least was funny because of how ineffective his bit ended up being.
I was trying to think of a good modern analogue, and the best I can come up with is this: imagine if Bob's Burgers did an episode where the kids ran into James Corden, *playing himself*, and they did a spoof on carpool karaoke. Would that still be funny to people in 2050?
(Side note: Bob's Burgers is smart with their cameos, as the celebrities play random one-off weirdos, so who the voice is becomes a fun bit of trivia but is not integral to the plot of the episode).
Scooby and the gang have a timelessness to them - partly because the franchise was so successful that they're still making new episodes, but also because of how clear their character definitions are, and how broad a lot of the jokes are. Plenty of original episodes have a joke or two that's distinctly 1970, but 40 years later you either miss those or find them dated in a charming way. To have the episode center not a character or a trope or a broad caricature, but rather a particular comedian's routine, just doesn't hold up nearly as well.
It's still enjoyable, and if you ignore the parts where we, the viewers, are clearly supposed to know what Winters is talking about, he turns in a solid performance.