I can't speak for Kermit's Swamp Years since I'd literally never heard of it until just now let alone seen it, but It's a Very Merry Muppet Christmas was basically somewhere in between Muppets from Space and Muppet Wizard of Oz.
It's a retread of the plot from It's a Wonderful Life, with the rare (I think) twist of actually following the whole plot, not just starting at the point where the George Bailey stand-in is about to end his life. So the first half revolves around a greedy banker, played by Joan Cusack, trying to prematurely foreclose on the Muppet Theater so she can tear it down and build a nightclub (funny how so many bankers and lawyers in movies are conveniently also in real estate) and the Muppets trying to raise enough money on the opening night of their big Christmas show to pay off the mortgage by the new deadline of midnight that night. (Yes, that's right, not only was the plot of the new Muppet movie stolen from The Country Bears, it wasn't even the first time they did it.) And of course failing, leading to Kermit's existential breakdown that drives the second half.
The NBC-produced special was full of cheap pop-culture gags (granted, if anyone can get away with pop culture parodies, it's the Muppets, but they're not at their best here), from Moulin Rouge to Steve Irwin to Fear Factor. There's a clever nod to the apparently-canonical events of The Muppet Movie in the second half, when Kermit is being shown how the others would have fared if he had never brought them together in the first place, and Cusack makes a good hammy villain. But it's clear that this is no outlying peak in the downward trend of the post-1990s Muppets.
They also made yet another Christmas special a few years later, Letters to Santa, which I didn't see all the way through because it clearly sucked and I was sick of the Muppets making another Christmas special every few years anyway.