^ I'm not convinced this is so. I'm not interested in celebrities, but I'm interested in the interactions.
Firstly, from this thread, it appears that the vast majority of celebs** know where their support comes from, and at least make an effort to be nice. Even most of the bad stories are balanced by others who say the same person was nice as pie on another occasion. It's difficult to say how many of these bad encounters were because the person was tired, stressed, rushed, or even just shy, which seems to affect a surprising number of performers offstage. A celebrity needs to be pretty obnoxious* for nobody to have a good word to say about them.
It would be necessary to have much more controlled experiments to scientifically come to conclusions about rudeness and obnoxiousness tendencies in famous people. Posher celebs are less likely to be met wandering round Tescos (I'd say a bad time to approach someone), some people had it harder than others in the scramble to make it, and that affects people in different ways. Some people just have much better manners than others, and it would be hard to pin down a tendency. As has already been mentioned, people change over time, too.
Finally, any fan who expects a performer to be similar to their stage or screen persona is just asking to be disappointed, yet people do! Some fans go as far as assuming TV villains to be horrible in real life! I'd love to see a thread of celebs telling stories of the most outrageous fans they have met... I can imagine there would be some shockers!
*Or simply have the surname Leslie, Windsor or the dreaded Black.
**By a good ratio of maybe 8-1