There are different kind of sequels and so different kind of problems.
You have the direct sequel continuing a story that was planned like that.
You have have direct sequels that were only made because the first movie was a hit.
You have different stories with reccurent characters, so you kind of judge each movie independently (the fact Indy 4 was not great does not makes Raiders less good).
And then there is.... The Terminator...! (yes again a "rant" of mine about Terminator)
Even if I am among the rare people who can enjoy ALL Terminator movies (on different levels of course!) I sort of agree with @"TM2YC" . The Terminator is a perfect loop story where the goal is not to change the future but to make it happen the way it should be (the whole story itself implies that nothing can be changed) . So as soon as you start to play with different timelines, you're fucked, because it is supposed to be a serious movie, not a a BTTF where you can do anything just for the fun of it.
What I like the most in T2 is the fact that we really don't know in the end if they changed the future or not. They tried, and that's the important message of the movie, humans being humans they have to try to change the future if they think they can (the opposite basis of the first movie where Reese just want the future to be as it shoud be). BUT I like that it is not said if they succeeded or not. We do not see alternate timelines in the movie, it is respectful enough of the original story.
So... I would say that even if The Terminator is a perfect movie on it's own, T2 is good "mirror" movie. My only "problem" being that the tone of the movie is way more family ortiented than the great horror / scifi / thriller first movie. The other T sequels I can enjoy them just like I would watch James Bond movies or fan films. We'll see what Cameron have in store for the next one...
All that to say that, no, I don't think of one single sequel/prequel that makes me like the original less. (if I can deal with Attack of the Clone I can deal with anything!)