This is something I feel is I think is a common opinion, but I actually believe it beaking didn't bring bad luck to the Great Valley. Not only was it never confirmed by the writers, but how often has the Great Valley also had bad stuff happen? In LBT 3, we get the water supply being blocked off. In LBT 4, Grandpa Longneck, the valley's leader, suddenly becomes terminally ill. Then in the next movie, all of the leaves are eaten by insects. Several movies after that, there's a huge blizzard and the population of the valley facing starvation, is followed-up in the next film with a monsoon AND an earthquake. A few films after that, a pair of sharp teeth manage to get into the valley...something Saurus Rock supposedly prevents.
Simply put, the Great Valley is not immune to bad luck. If Littlefoot had just accepted this, the last 15-20 minutes of LBT 6 could have been entirely avoided. Honestly, there was a great opportunity at the end for Littlefoot to understand that some kinds of bad luck is self-inflicted, but there are other kinds that just happen, and decide Saurus Rock didn't need to be fixed even after they got the tooth. But nope, they have to fix to "set things right", and even when Grandpa Longneck gently tries to assure Littlefoot that it probably doesn't even exist, Littlefoot still wants to be sure, to which Grandpa humors him. Because, you know, that's the lesson kids should be taught, to be afraid of things that are all in their head.
Aside from Cera's obnoxious cousins, this is the easily the worst thing about LBT 6. The only redeeming part of the kids being so paranoid is that we got easily one of the funnest songs in the series. If only it was enough to compensate.