Imo the only thing that got worse in the single player games was all the hand holding. You have to sit through 4 hours of getting told how shit works until you finally start playing.
Besides that and the stupid power creep, there has been a general improvement. Game mechanics make more sense, the quality of the graphics and the music improves and the plot has more depth as the generations go on. Pokemon designs are not as good as the first couple of gens but you can't blame them since it's a high standard and there's still always a bunch of worthwhile pokes each gen anyways. The post game gets steadily better as well.
Multiplayer wise, though, the first three gens were a complete fucking mess.