IMO, since
the timepicker will be used only in the admin area
, then YES, it is okay you place the 3rd party files into the admin/js and admin/css folders respectively.
With that, you (also other developers) can be able to clearly know where to find files that corresponds to their respective views.
Public Folder: For files that will be used in the frontend
Admin Folder: For files that will only be used in the backend.