Poetry is the art of the written word, and so reading from printed text is acceptable to my mind, and if done in animated fashion, with ad libs, etc, can also be engaging for the audience. Singing is about the sung word, and reading the lyrics whilst singing them simply distances you from your audience and creates musical wallpaper, over which the audience feels comfortable talking. Ignore your audience and they ignore you.
We?ve all heard Best Man speeches, where one inconfidently reads out a rigidly prepared script and the jokes fall a bit flat, and others where they seemingly improvise around a few basic prompt cards and they go down a storm. In the latter case, the key is practising beforehand until you know your speech well enough to be able to deliver it in an engaging fashion, and only need a few key words as a prompt to the next anecdote. All these one man comedy shows work on the same basis, though come over as fully improvised.