It's not an album, but I think the Star Wars Holiday Special probably jumped the shark even before Fonzie. That's the gold standard of bad television pretty much. But of course that wasn't a late-career blunder. It was an early-success misstep that didn't ruin the franchise, which would occur many years and many millions of dollars later.
Okay, I was wrong, Star Wars jumped the shark a year after Fonzie did. But it was more horrible by far.