All these games will easily run on the native resolution, but they won't really benefit from it as there do not use any HiDPI textures. Besides, I don't see much point running FTL full-screen, the game is designed for a window.
P.S. Dwarf Fortress Mac version is just beyond horrible, I recommend running it through Parallels.
No, they will not look blurry. Not any blurrier than a native 1200x800 display, anyway.
Its a quite tricky question, actually - 'blurry' would usually apply to some sort of high definition imaging, and yet FTL uses pixel art (I would say it looks 'blocky' ) and Dwarf Fortress is ASCII art. With don't starve I could't see much difference in image quality compared to the normal window mode.