I'll go ahead and make this clear since I don't feel I went into enough detail in my first post: I'm not saying that Littlefoot shouldn't have gotten angry at Cera. I would have expected him to. I'm not saying Cera was innocent either, because she was not.
What I am saying is that I feel Littlefoot was already getting hostile towards her before she insulted his mother, and I can't fathom why if I were to think of him as having the same personality he did in later movies. I can see him being determined and trying to urge Cera not to leave, and you could say he was doing that, but look at his reaction specifically. He's angry, he's hostile, and he's not asking her in a nice way not to leave; he just declares that her way was "wrong" despite having no way to know that for certain.
Yes Cera's insult was hurtful towards him, but it wasn't the worst insult she could have come up with, and when Littlefoot started badgering her, she didn't say anything further about his mother. Littlefoot attacking her for one insult is going a bit too far in my opinion and he could have handled his anger in a better way, for instance resorting to simply yelling at her to leave.
But again, I'm not saying Cera was in the right and I'm not saying Littlefoot should have gotten upset. I'm just not convinced that Cera's one insult puts her entirely at fault and justifies Littlefoot's violent reaction. Now if Cera used harsher words or continued to mock Littlefoot's mother, then I could see it ultimately snapping Littlefoot into attacking her.