I don't think you can do that to much extent. You can however:
- Sort your owned items by "Bought", "Bundles" and "Donated"
- I think you can ask the support if you want to remove something from your library - I am not sure about that one, though
That being said, the "big feature" of the library system is that you can actually create your own collections and sort your items that way. Whenever you claim an item, there will be a button to add and/or create a collection to put the item into. You can also access this window from an items itch page and/or your library by hovering over an item's thumbnail.
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All this is in regards of how it currently works. If your post was meant as a suggestion for adding to the current system, maybe you would like to post it in https://itch.io/board/580406/ideas-feedback instead.
Hope that helps.