Its not so much as the AI is bad. But after a point you start to learn the habits and such of the AI. So you can now predict it, making the AI seem very easy.
For example, the AI always rushes and/or attack with a large LF fleet every game, every first battle. So all you gotta to do from the get go is get LRF. Your first encounter with the AI is now easy/eaiser.
Over time you learn for lack of a better word to exploit against the AI. With Diplomacy, the AI is much much better yet it still haves is habits. Like the LF fleet. Just expect more numbers now.
Once you learn the personality of the AI you can fight 1vsX. Pre Diplomacy there were many who could do 1vs9 Unfair, while watching TV, while writing a thesis, while curing cancer, while ending world hunger. LOL But from what Ive heard Ive yet to hear somone win against 1vs9 Vicious. I think JJ has beat 1vs5-6. Anyhow, point is the AI is much better than it was and IMHO is great.