Hi @lexicon911,
The Downloads tab will show you the content the launcher reads in your Documents \ Electronic Arts \ The Sims 3 \ Downloads folder. If you remove content from that folder, that content won't be uninstalled, it will just not show on the Downloads tab anymore.
You could create subfolders in your Downloads folder and organize your downloads in those subfolders. The launcher won't read anything inside those subfolders, so that content won't appear in the Downloads tab.
Hope that makes sense, if I didn't explain myself well enough, let me know 😉
-Mai