"Not a decoration" means something is not for beauty or adornment but has a more important, practical purpose. It describes something that is functional rather than purely aesthetic, such as handles on a container that are essential for its use and not just for looks. Link
I am not the decorative type of person, I really do not fancy decoration. I think it is too elaborate. Being moderate has always been my greatest suite.
Sometimes, you can do decorations that are not looking shiny but have relevance to what you are doing. This is the major reason for the decoration, to give beauty and create awareness, thereby making the event or house look the essence of the decoration.
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