Of course the TR crew aren't meant to be serious. That was why I liked em so much. But yeah, they are really really going downhill...supremely. I mean, as annoying they are (showing up every single episode) at least towards the beginning, their cracks were humorous at least! Like:
Jessie: (irritated) Can't you let us commit a crime in peace?
Jessie: ARG! How dare that girl humiliate the twerp's Charizard that way!
James: (stupidly) Why, should she have humiliated it some other way?
Jessie & James: (sobbing) It's so sad...to imagine the feeling of rejection they're going through...
Meowth: You two don't have to imagine.
But now, their cracks are becoming lamer and lamer to the point where you just want to roll your eyes and mute the TV whenever they open their mouths. The dialogues were the biggest disappointment, and the cruddiness was added to because of the Voice Actors changing. I know I for one loved to hear James and Meowth's with their original VAs (James's really took the cake...in a good way).
In Advance Generation and Battle Frontier, they were dethroned by Harley anyway (who got the best dialogs of the whole season) so their lameness wasn't really noticed. Not to mention, they had an excuse...being new VAs and a new company taking over the rights and all. But with D/P, they're unbearable to the point where I've just stopped watching.
So yeah. Downhill. Supremely. By Far. :/