Two simple editors that work with devanagari text are CotEditor and TextEdit. The later comes out of the box, but I avoided it for a long time because it prompts you to open or create a new file each time it is launched. (Contrast this with the behaviour of Notepad on Windows) Thankfully, this is easily modified:
defaults write -g NSShowAppCentricOpenPanelInsteadOfUntitledFile -bool false
MacDown is designed for use with markdown files, and works with devanagari text. This makes it well suited for blogging. Atom is an open-source editor that works well unlike most of it’s brethren (i.e. Sublime Text and BBEdit).
Pages and Microsoft Word both handle devanagari text well, but are too heavyweight for simple editing tasks.