Hello fellow screentone enjoyer. Try out:
1. This board's Tegaki tool of course
2. CloudPaint (A bit of a bitch to handle on both mobile and PC)
3. Kleki (Works great on both mobile and PC, probably best one in the list
4. GaryC's noz.rip version (Not pixely though)
5.
Paint.NET (Has screentone brushes and fill bucket tool that can Paint areas with several types of patterns and screentones. Only "real" program on the list)
These are all browser-based though, don't think I've seen an actual program be able to make this type of pixely Flipnote-like screentone except
Paint.NET, unless someone made any brush packs for SAI, CSP, Krita, etc.
Hope this could help, if even just a bit