This is actually a complex topic. There is a loud group that prefers Disneyland keep their rides as original as possible. This position, as are others, is legitimate, especially for classic rides that Walt was involved in designing, and most especially those that have classic "stories" or characters. On the other hand, people like me like when things are changed up a bit, especially due to technological advancements, as well as, since I go there at least 1 or 2 times per month, to make repeat visits more interesting (something dear to Walt's heart, from what I understand).
For many rides, no one even brings it up. Midway Mania had new characters installed after Toy Story 3, and no one really complained. In fact, the ride is conducive to being updated or changed regularly, and I think most would agree that that is good. The biggest thing is that the changes are not drastic or invasive.
On top of not being very invasive, I think it works also if a) the overlay is good and b) it's short-lived. Space Mountain Ghost Galaxy lasts only 4-6 weeks, most people REALLY like the overlay, and it's not a drastic change of the ride.
One ride where it lasts a long time (almost 1/3 of the year), is highly invasive, and "good" is questionable is the Mansion overlay. First, it's obvious that the Mansion overlay is a boon for Disneyland. It's the most crowded you'll see the line for the Mansion, and they even use the FastPass system for it. Second, though, is that there are more vocal opponents of this than anything else.
Because I go year-round, it doesn't matter much to me. It does kind of break up the monotony, so to speak, though that is a harsh word, of riding it year-round for years. For others, though, it might take up too much of the year. One of the examples given here is that it's the only incarnation of the ride many see, since they only come at Christmas. Is that Disneyland's fault? I would argue that it is not. The biggest problem is that, unlike Ghost Galaxy, Nightmare is a Christmas/Halloween hybrid theme that only makes sense if it lasts that long. Instead of overlaying a ride for 10% of the year, it must overlay for almost 1/3 of it which is too much for many people. So, what to do?
I have always thought that Disney's best option would be either to a) build a Nightmare Before Christmas ride, or b) use Nightmare as an overarching theme for its Halloween and Christmas celebrations. This would give them the freedom to find different ways to sprinkle Nightmare in, and some of the time that could include the Mansion. If the theme is that Jack and everyone has taken over the park, they could do so much more with that and not have to overlay the mansion in an invasive manner for such a significant part of the year. They could, as suggested above, do it to the Mansion every other year, or every few years, and in the odd years find ways to sprinkle it in elsewhere.
IMO, I think updates/changes are good. Ghost Galaxy is a great example of a good, short, seasonal overlay that is not to invasive to a ride and adds a fun touch that keeps things interesting. Mansion/Nightmare is an example of something that a LOT of people like, but perhaps does TOO much to please a majority of Disney visitors. I would like to see changes in the form of improvements to rides. Wouldn't better animatronics make the Matterhorn that much better? What about Jungle Cruise changes? Whatever we may think about the Little Mermaid, it's better than Pinnochio. Wouldn't the Fantasyland rides get better with improved technology and animations? Just the minor changes on Snow White made a big quality difference.
Maybe Disney should find a way to sprinkle changes and themes in a less ride-invasive way and a more park-focused way.