What the ending needed
Yes, the ending had all the dogs dancing to the Romantics' "What I Like about You"--including the Duke's (that my nickname for him) very robotic robot dance--and then Duke farting in his owners' faces and then winking at us, the audience, which was a nice touch as such "fourth wall breakings" always make me feel more engaged with the characters.
But what the ending needed after that was a rap song that highlighted the Duke's adventures--with the appropriate scene replayed to accompany the music. You know, something like
Duke! You came from Kansas!
You're a dog, that's what the best friend to man is!
Taking a bath? No way, you're not having it--
Run down the stairs, draggin' Phil like an idiot.