Round my way the assumption has always been that Morning Glory is at least partially a song about drugs. "Chained to the mirror and the razor blade" seems to be a fairly blatant reference to cocaine use. "Morning Glory" can be read many different ways, but one of those is as a reference to the seeds of the Morning Glory plant, which contain ergot alkaloids and are thus hallucinogenic.
More than that though, it's a song about the sense of clarity and satisfaction and potential that can be felt in a day, when one wakes up, when one is walking in the sunshine, when one is stoned off one's tits. As such it is exemplary of the uplifting mood that characterises much of the music of Oasis. In the words of Noel Gallagher himself:
"Do you ever look at the sky and think, I'm glad I'm alive?"