Is there anything specific about Notepad++ that you need? Ports of software like that to AmigaOS are likely to be slow and clunky due to the large frameworks upon which they're built. For an Amiga native editor, what about
Annotate? It has user-definable syntax highlighting, source folding, and lots of other "modern" features in common with the likes of Npp or Gedit. It also has an ARexx port, which means it might be possible to add other features you need without having to update the editor itself.