Even though he is a side character, it's nice to acknowledge that he's not too bad.
I can understand that the other folk don't like him all too much being that Thicknose fills the children's minds with disbelief.
Even though I haven't watched the movie where he was first introduced, I can only say that he simply tells the kids about the wonders of life, along with teaching them on how to live to the best. It all ends to the children whether they want to believe him and take his advice or not.
There was lots of conflict between Mr Thicknose and Mr Threehorn. Being that there isn't much evidence for the things Mr Thicknose says, it makes sense how some of the dinos don't believe him. They just need to realize that he had lived longer. And if Mr Threehorn still won't listen, tell him to eat a rock.