The word "Clone" is, somewhat, misleading. A clone isn't like a copy you make on a Xerox machine (in which case, there are bound to be small errors - which keep getting amplified as the process repeats itself - resulting in a badly distorted image, several copies down the road). Although this is exactly how some people imagine the cloning process to operate - distortion and all!
What really happens in the cloning process is that you are not making a "copy" of anything at all - you are actually taking part of the "Mother" plant and allowing it to continue growing. Clones are just disconnected parts of the original plant; nothing more - nothing less. There is no degradation or change in clones because they are part of the same plant!