On the one hand we can take it for granted that until LBT 8 was produced nobody thought of Ducky as bein "unable" to get mad. Therefore we see quite a few frowns and scowls from her throughout LBT 1 to 7. Nevertheless I think there is a difference between the "mad" in LBT 8 and the cases we saw earlier.
In Spike's hatching scene for example I wouldn't think of Ducky's emotion to be any more than impatience.
In LBT 5 she was shocked at Petrie's statement that he wouldn't miss her (before she got the meaning that this meant they would stay together).
In the moment from LBT 6 you describe she is probably frustrated about Petrie's not very reassuring remark in a moment when she is rather scared.
In LBT 6 I saw pity with Dinah and Dana and reproach for Cera rather madness in Ducky's reaction.
There are several more examples from the earlier movies (e.g. Ducky looks clearly annoyed after Spike almost "ate" her along with a treestar at the beginning of LBT 5).
However, each single case had a major difference to what we saw in LBT 8. All the previous cases were just moments in which she felt annoyed, but she never ever held her possible grudge for more than a few moments. In LBT 8 we had a very lasting anger built up through a number of problems she had with Spike (being unable to sleep with him snoring beside her, his eating of her treestar, his jeering her during Mr. Thicknose's lesson (and Mr. Thicknose's presenting Spike as a positive example to her anyway), his behaviour during the game after that lesson...). Much more anger had built up in Ducky there over a longer space of time. She could no longer shrug it of the way she had always done on earlier ocassions. This I think is the difference between Ducky being momentarily annoyed in the earlier movies and being really mad in LBT 8.
Looking at all the cases in which Ducky might have been mad in the earlier movies but seemed to be very accepting (showing emotions like sadness, resignation, or even understanding and pity for whom she should be mad at) we get a long list.
In LBT 1 there are two scenes (just after they found the copse of trees and a herd of longneck causes a little "earthshake" and during their journey between the burning mountains) in which she urges Spike to get on, but she does so with desperation rather than anger.
In LBT 2 she never shows a sign of madness towards the eggeaters who tried to eat one of her unhatched brothers or sisters. Some scolding she utters at the sharptooth sounds defiant rather than genuinly angry.
In LBT 3 she seems to be the least angry about Hyp, Nod, and Mutt and most ready to support Littlefoot's suggestion of the bullies to play along with them at the beginning of the movies. Even the strange behaviour of the grownups doesn't seem to bother her as much as Littlefoot and Cera (which brings us to the question how Ducky's and Spike's parents and Petrie's mother behaved during the time). She didn't get mad at Spike for making it so hard for them to wake him up either.
In LBT 4 unlike Cera (of course) and Petrie (not quite so of course) Ducky doesn't utter any reservations against Ali. Nor is it her to point out that Cera didn't originally mean to come along on the rescue mission for Littlefoot.
In LBT 5 we have two twinges of anger both of which are very fleeting however. She doesn't show any sign of anger coming from the long miserable journey in the beginning (neither do Spike and Petrie, while the effects got Littlefoot and Cera (who otherwise might have grown kind of accustomed to their parents talk) into a fight).
In LBT 6 Ducky several times seems to act as kind of a calm counterpart to Cera who tries to blame her anger on others.
In LBT 7 Ducky of all characters (with the possible exception of Petrie) shows the least outright anger about Pterano, and about being abducted by him.
