I watched all the LBT films back to back over the past week and I can say for a fact that the animation of frames got better with each sequel (at least consistently starting with 6); though, as is typical with LBT sequels, some parts can seem Ok while others are amazing.
LBT 9, 10, 11, and 12 had great animation. Though in 12 the resolution had been decreased and the expressiveness of the characters were a little diminished, while in 13 the actions of the characters were less concrete (EDIT: but after watching more, some are just lazy; EDIT: woah they really lowered the quality 1/3 way through ha creepy). I think what people dislike are the style changes over the years rather than the actual frame by frame animation or the backgrounds. LBT films gradually became brighter and more 'friendly'. Backgrounds, especially in 10 and up, had lots of moving layers. The character designs have changed with almost every LBT. Each movie it seems the characters are represented a little differently. Looking at the Littlefoot Styles topic I made only proves the point. It's mostly noticable with Cera though. In the very beginning sequels of LBT, she didn't use her 'skin brows' when she was angry. They didn't exist. she had eyebrows on her eyes that went down when she was mad. However, in later sequels, here skin eyebrows would eventually appear. In some of the last sequels, she would be made to have bigger eyes and, lighter line colors, and other changes per sequel (like in 8 she had big cheeks that gave her a thinner mouth).
From what I read on LBT wiki, 2-4 were being made concurrently, so if you think all those have the same style and you wonder why later ones don't, that's because they were not part of the same 'team'. Also, I heard LBT was supposed to stop at LBT 6, but look how many things we learned about characters and all the characters we were able to see, and all the expressiveness of characters like Littlefoot and Ducky we would have NEVER seen had it not been for the sequels after LBT 6. Even in LBT 13, with the weird kiddish plot it has, I still saw good expressions and scenes.
I think the stylistic change over the later years and the fact that the older LBT fanbase was 'growing up' was what alienated some of fanbase by the last sequels. In fact, when I first saw 9, I didn't like it much; when I first saw 11, I was really upset by the voice change and the some of the floridness; and LBT 12 and 13, released around when I was 12 and 14, were released when I was no longer much of a real 'kid'; and so obviously did not fit as much my demographic anymore; and when I had watched them, I found LBT 12 not very memorable and LBT 13 really weird.
Watching the LBt sequels again 5-6 years later, and in order from first to last, I appreciate LBT 9 more now, I view LBT 11 in a happy light despite Littlefoot's voice change, I very like LBT 12 now, and-- well I'm watching LBT 13 at this very moment haha but yeah.
EDIT: I do understand the plots kinda became less "grand" in the later sequels, and maybe a little 'easier' for kids. Still, good movies lol
I wonder if some of the fanbase disliked the later sequels because LBT was gradually changing its demographic to a different generation WHILE some of the fanbase was likely more disapproving of the films because of the "coming of age" or "start of adolescence" time they were experiencing at the time... like me. I was at an age where I was wondering "should I stay or should I go?" and the LBT films become a little more child-catering didnt' help that fact
I think most people aren't lucky to have watched fun but somewhat serious early sequels like LBT 2-4 and so would have a much different view later on than someone who would have.
I guess.

Btw, just so you know where I'm coming from. LBT 2-5, some of 6 (could never get a hold of it) and then later 7-8 were my 'Classical' period of LBT. The first ones I watched and the ones I knew about for the longest time, mostly before I was 9.5 years old. I had lived in Ecuador from age 4.5 to 9.5. Don't think of that as just a coincidence though; I just felt older in general, especially over the next years.
Each LBT has its own charms :3