In the earlier seasons, D.W was also kind of insulting. I rewatched "Arthur's Treasure Hunt" one of my favorite episodes, and I was practically crying with laughter on the bit where Arthur dug up the Roman city. Another bit that I laughed at was when D.W asked Arthur if he was burying his report card. Then, I watched "Buster Baxter and the Letter From the Sea" and it was so boring and blatant. There was practically no humour, D.W was the most boring part of the episode, and the part where Buster cleaned up the beach seemed very goody-goody. A lot of episodes from season 14 are like that for some reason.
Why did the writers change D.W from the source of humour in an episode to the worst part of it? After the Time Trouble episode, she was basically a different person, until Baby Steps in Season 16, when she again insulted Arthur.