Pinta is not comparable to Paint.NET. Paint.NET is a very polished program and Pinta... isn't. While it's true that it is based on Paint.NET 3, the part they kept was the filter and image manipulation stuff and the part they dumped was the UI, and their (Gtk# I think) UI is not very good compared to even the WPF UI of the old versions of Paint.NET it was based on. Stuff like the selection handles being rendered on canvas so looking very broken when working on e.g. pixel art, or just highly zoomed in as the marching ants scale with the image pixel size.
If you need an open source editor for those use cases, go with KolourPaint or Krita. Krita is not quite as simple as Paint which is why KolourPaint might fit your use case better, but I've switced to Krita for my Linux image editor. Now if they could just bring back an on-canvas text tool...