From what I've read, John was heavily into LSD during the period of time from '67 through at least '69, so he tapped into some very heavy creativity. However, he had admitted that during that time, he was pretty laxidaizical about what was going on...losing interest in the Beatles and letting Paul take the lead. Yoko had also started influencing him a bit and when he wasn't in the studio, he was lolling around his home, daydreaming and attending art shows. It was a pretty passive period of his life, but he put out some of his greatest music then, yes!
I think John hit some highs and lows during the post-Beatles era...one of my favorite albums is "Walls And Bridges" which he made when he was away from Yoko, interestingly enough.
He wrote some beautiful and tender ballads for Yoko, even on "Double Fantasy", you can't take that away from him...nor his trademark acerbic wit. John ran the gamut from tender and sensitive to downright biting at times, but that's why I love his music...he wasn't afraid to show his emotions.