I originally watched this movie last Christmas. Surprisingly, they showed it multiple times on Comedy Network (basically Canada's version of Comedy Central): not exactly a network aimed at children. I'm a younger adult who's familiar with the muppets, but was born after their best years, so I wasn't sure exactly what to expect.
Don't expect an Emmy winning effort or anything more than a super-thin plot here, on those metrics the movie doesn't do much (it was originally made for TV after-all). However, it manages to get over these issues by having just a really fun Christmas atmosphere and some decent family-friendly jokes. In particular, Kermit and Pepe really drive this release. At times, for adult viewers, it is cheesy to say the least, but somehow with the Muppets even that seems forgivable. There are some great parts in this movie, starting with some great slapstick and overall comedy of how much damage the Muppets manage to do at the post office, to a great Sopranos reference (seems like the writers threw this in especially for adult viewers), to the humor of an actual airline that goes to the North Pole, to Whoopi Goldberg's interrupting of a heartfelt song to ask for money. As mentioned, the plot is thin, but it's enough to hold all the funny clips together. The songs are simple, but don't drag on and seem to work.
Perhaps the worst-part is the ending. As another reviewer noted, there was so much potential at the North Pole, but they seem to leave as soon as they arrive and it just feels underwhelming. Then there is a cheesy, lazy ending that really "wraps things up" out of nowhere. The movie could have been a little bit longer in return for a stronger ending. Additionally, some viewers may be disappointed that many of the Muppets only make appearances at the start and end of the film, it is primarily Kermit, Gonzo, Pepe, and Rizzo earning the screentime.
All of the above issues aside, I still give this 4 stars because while I never turned on the TV purposely to watch it, whenever it came on Comedy Network last Christmas, I never changed the channel and ended up watching it probably 5 times or so. And this year I've found myself searching it out again. Overall, the spirit, fun attitude and atmosphere, and Christmas cheer of this movie really manage to make up for its shortcomings.