Okay, so I finally managed to find a way to watch this one without having to pay for it, and here is my overall review:
Overall, it's not as bad as I expected. Granted, compared to pretty much all the other LBT sequels, this one is probably the worst, but not by as much as I imagined. From what I had heard, I was imagining it to be absolutely unwatchable (The kind of bad ovie you get on Mystery Science Theatre or Cinematic Titanic). But up till the Yellowbellies entered, it was okay. I'll admit they weren't as annoying as I expected (Iwas expecting the to be on par with Jar Jar Binks or any Rob Schneider character) , but their designs were still pretty bizarre. They look like no dinosaur I'VE ever seen!
The plot was pretty bog-standard material. No serious twist, no extremely dramatic moments, even the battle against the Baryonyx wasn't very entertaining. Also, the ending didn't really have that nice feel the other endings had, which ranged from IX's and X's undiluted shmalz, IV's charming flashback montage, heck even XI had that nice little camera rotation thing. But this one...nothing. The just..go home, and that's it.
Honestly, I think the only really redeemable factor of this film are the first and last song. Say So was pretty fun stuff, and I kinda like the way it segued into the next scene, and How Do You Know had a nice ring to it, especially the harmonising at the end. But to me, the message of How Do You Know seemed a bit confused. Was it telling us to trust our heart or listen to our elders or what?
So in conclusion: Pretty bad, definitely the worst of the series, but if you can find some way to watch it for free and skip through much of it, you may as well.
EDIT: After watching a couple more times, I do have a few more things to say: "Say So" would have been good if the segue INto the song was any good. But it just seems a bit too sudden.
Being a fan of rather immature humour (slapstick more than anything), I did find their antics kinda funny. But the belly-bouncing thing really didn't help. It just seemed wayyyyyy to weird. I think that if they just made the Yellow-bellies look fat (as opposed to now, where they look pregnant), and maybe toned down their enormous butts, they might be redeemable characters.
And having finally plucked up the steel to listen all the way through "Yellow Belly Bounce", I am kind of amazed at how catchy and fun I think it is. Granted, it basically serves nothing for the plot, but it's just a big, dumb, fun catchy little song, plus, apart from the dance-number in the middle with Spike and Foobie, the animation was pretty good too. That's basically how the LBT songs work for me. If they have a good rhythm to them, good vocals (especially if they harmonise) and good instrumentals, it will quickly become popular with me. The message usually is second in terms of importance.