With respect, that's not really much of a smart folder. Smart folders are useful when they collect files from various locations on your system and present them as though they were in one folder. If the files are already in a single folder, there's really no point creating a smart folder.
If you have subfolders in Dropbox, and you actually want to see ALL the files in ALL subfolders gathered in a big list, a smart folder can do that for you. In the Finder, go to your Dropbox (normally there's a shortcut for it in the sidebar). Click cmd-F to open a search box, and click "Dropbox" at the top next to "This Mac". In the search box at top right, type an asterisk (this just means "all files"). The only search criteria should be "Kind is Any" (you can change or add to this if you like, to narrow down the file list). Click Save and name the new Smart Folder. Ticking "Add to Sidebar" keeps it in every Finder window.
Hope that helps!