It’s a perfectly valid description of a particular phenomenon. It’s that sad and depressed feeling you get when you realize that no matter how great and majestic and important something is at the time, in time it’s going to pass. Just like the poem – eventually, time kills everything. It’s just that rotting statue of Ozymandias, a once-great statue, and now a broken-down piece of marble in the desert. So you get a depressed feeling because it gives you a sense of the futility of life, that all that you’re working for, and all the things that seem so meaningful, are nothing.